If you ripped your CD's to a lossy format like mp3 when you put the music on your iPod, a lot of the original data was discarded in order to create smaller files. You can convert those mp3 files to a lossless format but you won't recover the lost musical information, you'll just create a file that sounds the same but takes up more space.
If you want to have the same quality as the original CD you'll have to rip it again into a lossless format.
If you want to have the same quality as the original CD you'll have to rip it again into a lossless format.