In anticipation of purchasing a new Mac, and as a result, a new iPod, I've been trying to get my iTunes affairs in order. Up 'till now I didn't think I needed iTunes or an iPod -- and I liked Windows Media Player alright. But the Mac calls for an iPod, which works with iTunes. So I had to transfer all my Napster-purchased music into iTunes.
This is what I learned:
1. To play iTunes-purchased music in WMP, burn it to an audio CD and then rip it as an mp3 file. Add the new files to your library. Done. Beautiful. And now you have a backup, too.
2. To play Napster-purchased music in iTunes, burn it to an audio CD, rip it as an mp3 file, then re-enter all of the id3 tags, and then you can add the new files to your library. If you rip with iTunes, you'll need to enter ALL of the tags. If you burn and rip with WMP, you'll only need to re-enter the album and artist info.
I wasn't able to find this information anywhere on the internets, so there you have it. My contribution to the universe's collective knowledge. If you know of an easier way, keep it to yourself, because I already did it all.
Moral of the story: buy everything from iTunes. Apple wins again.