Using getpart with docx subtemplate inclusion

It would be a nice feature to be able to use .docx files as subtemplates, especially if getpart could be used with it. This would allow my subtemplates to be maintained in Word without having to separate out the document.xml, rename it, and upload it separately. Maybe the performance hit would be too high to be worth it, but if not, this would be a handy feature.

Insert a docx inside another docx is quite difficult without M Office  because styles and other inner objects that are saved inside each documents can be different.
Thus, the inserted XML will have corrupted references.
I have not the solution yet.