Hi Thomas,
Yes, one would think that this would be easy.
However, PresentationML is significantly different from WordprocessingML, as their design goals are so different, so you can’t use PresentationML directly in a DOCX.
Further, I have done no work on converting PresentationML to a form that can be directly used in a DOCX. Someone else out there may have, but I know of no open source project to do so.
If I had your problem, the way I would proceed is to automate PowerPoint to save the presentation to another form. One of the options when saving in PowerPoint is to save each slide as a JPG or other graphic form. PowerPoint then creates a directory with a JPG for each slide in it. I would guess that there is an option when automating PowerPoint to do this, but I have never done so.
Once you get a number of images, one for each slide, it is then straightforward to insert those images into a DOCX.
Best, Eric