Yes, you can, and it's easy. It involves burning the song to a CD and the reimporting the song from the CD. There will be a slight quality loss in this process but most people would be hard pressed to hear the difference.
Here's what you do:
- make a new playlist
- add the song(s) you want to convert to the play list
- in iTunes preferences make sure that burning (in the Advanced section) is set to audio CDs
- highlight your playlist
- insert a blank CD
- click burn
- after the burn is finished, the new CD will mount and show up in iTunes
- in iTunes preferences make sure that importing is set to what you want (MP3, ACC, etc.) and consider upping the bit rate to compensate for loss of quality*
- highlight your newly burned CD and then click the import button
After the import is complete, your iTunes tracks will no longer have any copy protection on them. Enjoy.
*I recommend setting the bit rate to 192kbs on tracks that you're converting in this manner, doing so will make it very hard to tell the difference between the converted track(s) and the original(s)