GarageBand can handle MP3s natively, just drag the MP3 into the track window and drop it there. You could then add crossfades and effects to the mix. Not sure how long a song can be in GarageBand, so marathon mixes may be a problem.
Check out info below for track length (I have not tried this yet):
The problem is that GarageBand (like its bigger sibling Logic) is, at heart, a sequencer, not an audio recording program. This means that it's measure-based, rather than time based.
Result: Maximum recording time is determined by number of MEASURES, not by anything else.
Create a new song, and set tempo to 40 bpm instead of the default 129 bpm. You will now be able to record 33 min x 3 = 100 minutes. If you set the time signature to 6/4 time, you get an extra two beats per measure, meaning your maximum time increases by half again to almost three hours.
Note: You must do this when you CREATE the NEW song. Changing this info on an existing song will apparently not work, for whatever reason.