That's technically possible, but result usually isn't good. The thing is, if you change speed (tempo) of an audio file, then pitch changes -which is something you don't want. Some audio editors have feature to change speed without changing pitch, but that's usually only usable at relative small speed changes.
What you look after (I think) is the following: you save your (slower tempo) playing as midi file. After that you play that midi file at higher (correct) tempo. I assume it's not possible to play midi file and at the same time record it as mp3 -so you'd need to use main outputs on backside of keyboard for recording.
Bogdan
