AllStarAirSoftLLC talks about HPA tapping your gas blowback magazine or putting HPA through your gas magazine using an external HPA tank as power source. Is it worth it? It depends, though for us we prefer using such for target shooting rather for actual skirmishing. If you don't mind slower magazine changes during games, then you might find this economical in the long run.
Also included is a video on how to HPA Tap your GBB magazine.