For the impatient this is how you do it:

Extract the Dynamics 365 generated template from zip and add the CustomXML-part to your existing template.

I couldn’t find a post on how to create a new Dynamics 365 Document Template using an existing document or template. I’m sure many organisations have an existing template with all the right formatting, headers and footers they would like to use.

Knowing how .docx-files are structured and how Dynamics 365 stores the metadata it was easy enough to figure out. I thought I would share how with you.

Step by step

1. Create a new blank template from Dynamics 365 the way you would normally do it and save the file to your computer
2. Change the extension of the file to .zip and extract it
3. Open the existing document you would like to use with Dynamics 365
4. In your Word template click Developer > XML Mapping Pane
   
5. In the drop down menu select (Add new part…)
   
6. Navigate to your extracted template and select the xml-file (item.xml) under the CustomXML-directory
7. Proceed as you normally would with a blank template

Note that this is might not be supported, but seems to work just fine.