Avoid Hidden Mapping

1. First of all, create each field for the 2 entities with the same data type.
2. Then, create a plugin on the Order Detail triggered on the CreateMessage.
   

   /// <summary>
   /// Initializes a new instance of the <see cref="SalesOrderDetailPlugin"/> class.
   /// </summary>
   public SalesOrderDetailPlugin()
       : base(typeof(SalesOrderDetailPlugin))
   {
       base.RegisteredEvents.Add(new Tuple<int, string, string, Action<LocalPluginContext>>(20, "Create", "salesorderdetail", new Action<LocalPluginContext>(ExecuteSalesOrderDetail)));
   
       // Note : you can register for more events here if this plugin is not specific to an individual entity and message combination.
       // You may also need to update your RegisterFile.crmregister plug-in registration file to reflect any change.
   }
   

3. Check for the Parent Context Message to be ConvertQuoteToSalesOrder and replace the copy the data from the Quote Product
   

   protected void ExecuteSalesOrderDetail(LocalPluginContext localContext)
   {
       if (localContext == null)
       {
           throw new ArgumentNullException("localContext");
       }
   
       var context = localContext.PluginExecutionContext;
       Service = localContext.OrganizationService;
   
       // Check the parent message
       if (context.ParentContext != null && context.ParentContext.MessageName == "ConvertQuoteToSalesOrder")
       {
           // Get the source quote ID
           var quoteId = (Guid)context.ParentContext.InputParameters["QuoteId"];
   
           // Get the Sales Order Detail which will be created
           var currentSalesOrderDetail = context.InputParameters["Target"] as Entity;
           if (currentSalesOrderDetail == null) return;
   
           var currentLineItemNumber = currentSalesOrderDetail.GetAttributeValue<int>("lineitemnumber");
   
           // Retrieve the data from the quote detail
           var relativeQuoteDetail = getQuoteDetail(quoteId, currentLineItemNumber);
           if (relativeQuoteDetail == null) return;
   
           // Copy the data from the quote. Notice the data type.
           currentSalesOrderDetail["new_field1"] = relativeQuoteDetail.GetAttributeValue<string>("new_field1");
           currentSalesOrderDetail["new_field2"] = relativeQuoteDetail.GetAttributeValue<DateTime?>("new_field2");
       }
   }
   

// <copyright file="SalesOrderDetail.cs" company="">
// Copyright (c) 2015 All Rights Reserved
// </copyright>
// <author></author>
// <date>11/06/2015 09:43:20</date>
// <summary>Implements the SalesOrderDetail Plugin.</summary>
// <auto-generated>
//     This code was generated by a tool.
//     Runtime Version:4.0.30319.1
// </auto-generated>

using System.Collections.Generic;
using BackToTheCrm.Entities.Extension;

namespace BackToTheCrm.Plugins
{
    using System;
    using System.ServiceModel;
    using Microsoft.Xrm.Sdk;
    using System.Linq;

    /// <summary>
    /// SalesOrderDetail Plugin.
    /// </summary>    
    public class SalesOrderDetailPlugin : Plugin
    {
        #region attributes

        IOrganizationService Service = null;

        #endregion

    /// <summary>
    /// Initializes a new instance of the <see cref="SalesOrderDetailPlugin"/> class.
    /// </summary>
    public SalesOrderDetailPlugin()
        : base(typeof(SalesOrderDetailPlugin))
    {
        base.RegisteredEvents.Add(new Tuple<int, string, string, Action<LocalPluginContext>>(20, "Create", "salesorderdetail", new Action<LocalPluginContext>(ExecuteSalesOrderDetail)));

        // Note : you can register for more events here if this plugin is not specific to an individual entity and message combination.
        // You may also need to update your RegisterFile.crmregister plug-in registration file to reflect any change.
    }

        /// <summary>
        /// Executes the plug-in.
        /// </summary>
        /// <param name="localContext">The <see cref="LocalPluginContext"/> which contains the
        /// <see cref="IPluginExecutionContext"/>,
        /// <see cref="IOrganizationService"/>
        /// and <see cref="ITracingService"/>
        /// </param>
        /// <remarks>
        /// For improved performance, Microsoft Dynamics CRM caches plug-in instances.
        /// The plug-in's Execute method should be written to be stateless as the constructor
        /// is not called for every invocation of the plug-in. Also, multiple system threads
        /// could execute the plug-in at the same time. All per invocation state information
        /// is stored in the context. This means that you should not use global variables in plug-ins.
        /// </remarks>
        protected void ExecuteSalesOrderDetail(LocalPluginContext localContext)
        {
            if (localContext == null)
            {
                throw new ArgumentNullException("localContext");
            }

            var context = localContext.PluginExecutionContext;
            Service = localContext.OrganizationService;

            // Check the parent message
            if (context.ParentContext != null && context.ParentContext.MessageName == "ConvertQuoteToSalesOrder")
            {
                // Get the source quote ID
                var quoteId = (Guid)context.ParentContext.InputParameters["QuoteId"];

                // Get the Sales Order Detail which will be created
                var currentSalesOrderDetail = context.InputParameters["Target"] as Entity;
                if (currentSalesOrderDetail == null) return;

                var currentLineItemNumber = currentSalesOrderDetail.GetAttributeValue<int>("lineitemnumber");

                // Retrieve the data from the quote detail
                var relativeQuoteDetail = getQuoteDetail(quoteId, currentLineItemNumber);
                if (relativeQuoteDetail == null) return;

                // Copy the data from the quote. Notice the data type.
                currentSalesOrderDetail["new_field1"] = relativeQuoteDetail.GetAttributeValue<string>("new_field1");
                currentSalesOrderDetail["new_field2"] = relativeQuoteDetail.GetAttributeValue<DateTime?>("new_field2");
            }
        }

        /// <summary>
        /// Get the data of the quote product
        /// </summary>
        /// <param name="quoteId">ID of the parent Quote</param>
        /// <param name="currentLineItemNumber">Line number</param>
        /// <returns></returns>
        private Entity getQuoteDetail(Guid quoteId, int currentLineItemNumber)
        {
            string fetchXml =
                        string.Format(@"<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>
                          <entity name='quotedetail'>
                            <attribute name='productid' />
                            <attribute name='new_field1' />
                            <attribute name='new_field2' />
                            <filter type='and'>
                              <condition attribute='quoteid' operator='eq' value='{0}' />
                              <condition attribute='lineitemnumber' operator='eq' value='{1}' />
                            </filter>
                          </entity>
                        </fetch>",
                        quoteId.ToString(),
                        currentLineItem
                    );
            List<Entity> results = Service.RetrieveMultiple(new FetchExpression(fetchXml));

            Entity quoteDetailToReturn = null;
            if (results != null)
                quoteDetailToReturn = results.FirstOrDefault();

            return quoteDetailToReturn;
        }
    }
}