assuming this is for your own use, not commercial (or otherwise violating copyright)
You can do it by burning an audio CD from iTunes, then re-importing (extracting). I'd recommend AIFF for FCE/P. Shouldn't be a problem importing using iTunes, but becomes more of a file management issue (dups and/or different formats). If you have Toast, you can extract the audio off the CD without adding to your iTunes library. (if you have a RW, you don't even need to waste the blank). As a side note, you should have backups of your purchased music anyway. Personally, I burn both audio CD and data (AAC's).