You can get LAME, which does various mp3 compression types on a standalone basis, and run those on the files to get mp3 versions. You would end up with both WAV and MP3 versions on the HDD and iTunes would presumably see both. Maybe you could have iTunes filter them in some way?
You should have enough storage to store both WAV and MP3--400 CD @ 450 MB/CD (WAV) + 45 MB/CD (alt preset extreme MP3) = 200 GB total, well below your total 160 GB x 2.
You should have enough storage to store both WAV and MP3--400 CD @ 450 MB/CD (WAV) + 45 MB/CD (alt preset extreme MP3) = 200 GB total, well below your total 160 GB x 2.