If it was your music, you could convert iTunes-recognised formats in iTunes, through the Advanced menu. What you convert to will depend upon what format and bitrate you have selected in the Importing section of the iTunes Preferences. Of course, this doesn't apply to DRM music and OGG, let alone music that's not technically yours. 🙂
Why do you want to convert it though? Going from one compressed format to another will only make the overall quality suffer.