Yes, it can be done. It just gets complicated.
Convert the mp3 to a .wav using music match jukebox.
www.musicmatch.com
Then use sound recorder (which I believe comes bundled with windows, I've always just had it) to edit down that .wav and convert it to a much lower quality.
Then use WSC-MA2-SMAF (Ring Voice) at
http://smaf-yamaha.com/tools/downloads.html
To convert the .wav to an .mmf.
If the .wav is too large or too high quality it won't work.
Have fun!