The real chore is to come up with an adapter that will fit the body tube of the pump gun, to allow you to feed CO2 into it. The pump pneumatic guns typically have a smaller diameter pump body than the CO2 ones, so the standard bodge of drilling and tapping a CO2 cartridge won't work. Usually.
Not complicated to make, if you have a lathe, but not trivial without.
Once you can secure that adapter into the body tube, then you need to find the fittings to get CO2 from a tank to the valve.
Oh. It won't work on a blow open type valve at all (OK, technically it could, but if you are asking, just stick to 'no'), you will need to start with a strike-to-open type valve in the first place.
Grab a paintball magazine at the magazine stand, and look though the adverts for hardware. That should give you a pretty good idea for what is available.
Take a look at the Mac1 Airguns site and see what he has for adapters, too.
Cheers
Trev