Just download the music from iTunes, burn it to a CD, delete the files off your computer and then rip the CD back onto your computer.
It's a hassle, very time consuming, and I'm fairly certain you lose sound quality by decompressing it then compressing it again.
Buy DRM-free and you don't have to worry about it ... as long Apple is selling 5 billion+ songs, they'll never be pressed to go DRM-free. iTunes has no trouble importing DRM-free MP3's to your iPod.