How can I save a Pages document as .docx? I've tried to save a pages document as a *.docx for Microsoft Office.

Pages is able to open, edit and (auto-)save existing *.docx documents.
However, there is no option to export or save a duplicate as *.docx?


Is this a missing compability feature and are there plug-ins for pages to add that functionality?
This does seem to be a missing feature. As I see it, you have two options:


Export as *.doc using File -> Export. While not ideal, I imagine almost everything that supports *.docx also supports *.doc.
TextEdit is able to save as *.docx. Of course this is only really an option if your formatting is simple.