You would be much better served to work within the constraints of the existing protocol if possible. I'm sure that could be done with IMAP which already supports UTF-7 encodings. You'd likely have to do a mapping of some kind. Good luck with POP.
Please don't get me wrong. An extension is possible and maybe is the right thing to do. But please don't pretend that it is either simple to engineer or (more importantly) to deploy, because they aren't. The same goes for SMTP as well ... even more so.
--Paul Hoffman, Director --Internet Mail Consortium