[Matt 'Maverick' Watts]

I strongly suggest you talk to a qualified paintball airsmith. They are intimately familiar with hpa and hpa systems and can help you with what you want to do.

If you're thinking of replacing the propane storage space in a GBBR mag with high pressure air, forget it. The magazine storage space cannot handle the pressure of storing high pressure air.
If you want to put a fill nipple on the GBBR mag and attach that to a remote line/hpa tank setup and use the GBBR mag gas storage space as part of the air system, it MAY work IF your mag valve can be adjusted to hold the proper pressure AND your hpa tank can be adjusted to output at a correct pressure for you application.