Yes and no.
I copied the original Word- and RDLC-documents into my project folder and then rebuilt the project, so that the XML-fields were inserted in the Word-document.
To give you a bit more information, please see the code I have entered below.
I have given my report ID 50200 and the name "Sales Order Confirmation".
report 50200 "Sales Order Confirmation"
{
RDLCLayout = '.vscode/SalesOrderConf/SalesOrderConf.rdlc';
WordLayout = '.vscode/SalesOrderConf/SalesOrderConf.docx';
Caption = 'Sales - Confirmation';
DefaultLayout = Word;
PreviewMode = PrintLayout;
WordMergeDataItem = Header;
dataset
{
dataitem(Header; "Sales Header")
{
...
It is almost exactly the same as the original report (1305), but I have added some columns in the dataset:
...
column(Shipping_Agent_Code; "Shipping Agent Code") { }
column(Shipping_Agent_Code_Lbl; FieldCaption("Shipping Agent Code")) { }
column(Shipping_Agent_Service_Code; "Shipping Agent Service Code") { }
column(Shipping_Agent_Service_Code_Lbl; FieldCaption("Shipping Agent Service Code")) { }
...
When I build the project, the added columns are inserted in the Word-document as XML-fields.

When I publish the project to the Sandbox and try to import the Word-document to report 1305, I get the following error message.
This is understandable as the report ID of my custom report is 50200.

However, when I generate the report with my custom report, all the fields except for the added fields are inserted correctly.

And when I export the Word-document, my added XML-fields are gone.
Sorry for the long post, but how can this be?