AFAIK, there is no easy way to do this in Pro Tools. I'm not sure about other DAW's, but you should check Ableton/ and Fruity Loops. The way it gets done in PT depends on whether you want the mp3 to follow the click, or want the click to follow the track(what was the mp3 before import). Getting the click to follow the track is probably the better option so check youtube for help on "creating a tempo map in pro tools"(without the quotes). Now if the track was recorded to a click, then it could be as simple as using the TAP function to tap the time while listening to the music, maybe with some experimenting on the BPM setting to match perfectly(again, youtube is worth checking for tutorials). Getting the track to follow the click gets really involved(using Elastic Audio) and the results might not be very good(because Elastic Audio can often cause lots of artifacts that garble the sound).
