Convert prius to ngv

yes, that sure is possible, but as a ex auto mechanic and ex shop owner and someone who understands the electronics and computers well i'd say that cost of making this system work properly with Natural Gas (so that you do not trigger any of engine fault codes) would be enormous.
You would spend so much time debugging and reprogramming the system that it would be crazy.

i see prius synergy drive as a fine tuned orchestra where each component (ICE, motor1, motor2, inverters and other stuff) work their part with precision as the musicians in an orchestra would.

so, imagine if one of them decides to play "his own notes?!

conductor would not be happy with that musician, just like engine ECU (car conductor) would not be happy when engine should rev at say 2000 RPM at 20% of throttle at 190 degree temp, but it only revs at 1200 RPM while the evap system is going heywire sending even more confusing data to the ECU. I could imagine that OBD would pickup that 5 or 6 sensors are 'bad', as it has no idea that you are now injecting natural gas into ICE.