Technically you can. There are programs (and why they'll exist I'll never know) that will take the mp3 back to wav and then automatically make a flac of it. It, of course, retains the quality loss of the original mp3 file.
This transcoding is more commonly seen on networks (like the old oink) that have bitrate requirements and someone trying to re-encode 128kbps mp3s up to 192 or whatever. The double mp3'ing of the music actually, in that case, has a multiplier effect on the quality loss.
