I answer very late
with some info that can help (a little)
but which has major drawbacks
OnfyOffice Desktop installs its models in a folder of the application itself.
under linux for example
(<= attention to your language)
There are 3 files new.docx new.pptx and new.xslx
It is not possible to directly edit these files in the folder.
but you can create new ones and replace old ones.
At the next opening you will have your own models.
But! During updates OnlyOffice will overwrite your files and you will have to start over.
If R&D could add a “templates” folder
with in “File” a Templates section which from the dotx docx potx pptx files … present in this folder created a new document.