Footnotes in docx conversions popup automatically, at least in the calibre viewer. See the demo docx file
https://manual.calibre-ebook.com/con...-specific-tips
They arent marked as footnotes using epub specific markup however as the docx input plugin doesnt do that. It might be possible to have it output role="noteref" which is the modern equivalent of epub:type="noteref" open an issue about it so I dont forget.