To duplicate the track you can just select the audio track you need and just copy the track. The next thing you do it to paste that track into the blank audio track.
However you will still have a mono track so using the PAN function will yeild approximately the same result. If you are using a track that is primarily music or sound effects you can sometimes insert a one, two or three frame delay from the original track and simiulate a stereo track. If you try this you can put the original track on channel Left with a hard pan and the delayed track on Channel Right and you can hard pan that opposite the original track as well. Then listen and experiment with both the delay and the amount of pan given to each track. You can arrive at a happy medium that sounds much like an original stereo track.
Be careful though – If you do this to voices you can end up sounding like the Cylons from the original Battlestar Galactica series. SO that is not something I recommend. Enjoy.