There is only one way I know of to do this, but it takes a long time:

First, make a playlist of all the songs you wish to unlock.

Second, burn all of these songs to a CD. You can't save them to the hard drive, you have to burn them to a disk.

Third, when you finish burning the CD, the CD will now appear with its own playlist, right? You'll see where you can eject it from the side panel and everything.

Okay, so you have the NEW list open (not the new playlist, the songs displayed on the CD), right? So now RIGHT CLICK on any one of the songs on the CD, and click on "CONVERT SELECTION TO MP3".

Now, this song is automatically being transferred BACK TO YOUR LIBRARY, but in unlocked, .mp3 format. Now you will have two versions of the same song. To find the correct song, just go to "GET INFO" on one of them. The one that has the format .M-Peg or .mp3 or the like, will be your unlocked song.

If anyone knows of a faster way, share!