Is it possible to export just the drum track of a song to my PC in mp3, WAV or some kind of audio format that I can use to jam over without actually playing the game?

No.

Yes, it is but not with cDLC's as they don't have drum tabs.

 

But you can load gp5 or gpx files from ultimate guitar or songsterr into Guitar pro, and save the drums as an audio in a pretty good quality.

The drums you can hear in my melodic sequences exercices are self made in Guitar pro that way.

 

You may also be interested to search the net for backing tracks, or multi tracks,

some people ripped tracks from guitar heroes or rock band series into separated tracks of drums