FYI - iTunes (the good folks at Apple) figure that if it isn't already an MP3 of some version, then you don't have copy rights and thus, don't have the right to make a copy - that's why it blocks making mp3's from AIFF, etc.

BIAB does a great job of exporting an mp3 file and these are easily dragged/dropped into GarageBand where you can export it to an mp3 with different compression resolutions. However, if you plan to do that, it's better to retain the audio quality by exporting it as AIFF in BIAB first, then make the 'custom' mp3 settings/output with GarageBand.

Incidentally, for kicks, I exported a few tunes using BIAB with various solos and trading 4's, then recorded the head and my solos with GarageBand, sent the file to some bandmates/and or played the CD - I didn't tell them it was BIAB. They responded saying things like:

"Wow, I'm amazed how much better we sound on recording versus when we play"
"I know that's your guitar, our drummer and bass, is that really me playing the piano? I don't remember playing those lines"

Of course, I fessed up, they laughed. I wager they're placing orders for their copy of BIAB.