@Colin: In order to be able to upload anything into D365 from Excel follow the following steps:
Step 1: Open Microsoft Excel and Navigate to the “Insert” Tab and Click on “My Add-ins”

Note: For the add-in to be available in excel you must have used the "edit in excel" functionality once.
Step2: Click on the Microsoft Dynamics Office Add-in.

Step3: You will see the following Add-in in excel: (It might ask you to sign-in the credentials will be the same which you use for D365)

Step 4: Click on Add Server Information and paste the following link into the Server URL then Click “OK”
https://<tenant url>.financials.dynamics.com/
Step 5: Click on the Settings button.

Step 6: Select the Company ID after selecting Click “OK”.
Step 7: Now Click on Design

Step 8: Click on Add Table
Step 9: Select the Table you wish to select.

This list will contain all tables in D365, some commonly used tables are:
1. Account Schedule
2. General Journal
3. Customer Card
4. Vendor Card
Step 10: After you select the table, you will be required to select the fields.
Step 11: After Selecting all the fields click on Refresh to update the existing data present in the table.
Step 12: The Data present will automatically be reflected.
If you have entered new data and accidently click refresh you might lose the newly entered lines.
Conclusion: This feature can be a little tricky to use but once perfected can be used for mass entry of data in any tables, there are some limitations with respect to non-editable fields. Also I usually enter one record manually into a table then refresh to see which fields I need to use.