I'm assuming you have older encrypted music from iTunes that you want to reimport. Depending on which version of iTunes you currently have, I would try to use a software program called Requiem.
Requiem is a program that decrypts your music (and videos, though not rentals) the same way that iTunes does, losslessly, and copys the decrypted version to a new file. It works great so long as the version of requiem you have is compatible with the version of iTunes you have. Good luck.