Anyone bulk feed a Benjamin Rocket .22

I was given a Benjamin Rocket .22 awhile back and told that it didn't hold air. I tested it out yesterday, (found that it leaked), tried to disassemble it, but couldn't get to the valve. So in a bit of desperation I squirted a big smear of silicon grease into the tube, loaded up a CO2 bulb and whadaya know? It sealed and has been holding a charge for hours now.

So here's the new problem I face. I've got this kind of neat little CO2 pistol that only takes 8g CO2 cartridges. Given their price and availability, that means when the few I've got are gone the gun will probably never be shot again. I slipped in a 12g powerlet, but it was pretty obvious it couldn't be easily adapted.

So I'm wondering if there's some sort of end cap available for these things that would allow for a bulk fill. The valve is already sealed to the tube, so all it would take is a cap. I checked the cap from my 2240 and the threads are different.