The only high quality way I can immediately think of would be to Edit > Duplicate the track you want to have a changing pan, pan one copy hard left and the other hard right, then press F2 to make a different volume envelope for each track. For example, fade in the left-panned track and fade out the right-panned track which will make the track seem to pan from right to left over time.
You could also record the track as you play it while modifying the pan, then export the recorded track. That would be slightly lossy.
Gale