Depends on how picky you are about your sound quality. Transcoding from one lossy codec to another is always going to involve some degradation in sound quality.
The way you do this in iTunes is to go to iTunes Preferences (assuming you're using the Mac vesion), go to the Import settings and select mp3 for your import format. Then select the songs you want to convert, go to main menu, click Advanced, and you should have an option for "Convert Selection to MP3".
