I don't think its possible... Midi files (I think) are like a text file that the computer can play, but mp3 is sort of a raw sound file and there is isn't a way for the computer to "write down" the music notes from the sound... This is all speculation, I'm not quite sure.
EDIT: Ok, they do exist... google "mp3 to midi converter" and there'll be some free trials you can try.