Okay, out of curiosity I looked into the docx source xml. Automatic page-breaks are stated there.
<w:lastRenderedPageBreak/>
is the term used. To get to the source, you have to unzip the docx and go to the "word" folder inside it. There should be a document.xml. You could open it with something like Notepad++ and do a search and replace to insert some unique text before each page-break and the do a search and replace for this text in your epub. Be aware of course, that the pagebreaks in this new docx are on different positions (of course), but that is unimportant for the epub. Just be sure to work with a copy, not the original file.
Much hassle for something I still don't realy see the point, but it is possible after all
