Can you fill a paintball HPA tank with an air compressor?

Only if it can produce 3000 or 4500 PSI . Take it to a paintball store and normally they give you a fill for a dollar or so. If you are playing and you need to refill, go to the refill station, attach the fill hose to the air nipple while the tank is on the gun, press the lever or button until it is filled at the correct pressure.