Presuming they aren't mp3 format files:
iTunes: How to convert a song to a different file format - http://support.apple.com/kb/HT1550 - including information about different formats and discussion about compression.
In iTunes11: File > Create New Version
In iTunes 11 "Create..." is in the File menu, not the Advanced menu.
Topic: Need to convert mp4 to mp3 - Zevoneer's post on converting songs - http://discussions.apple.com/message.jspa?messageID=9244740
Converting will place duplicates in iTunes but if you look at the kind you will see one version is mp3. Drag those files to the USB drive.