generally speaking FIRST has never allowed air pressure to be applied to anything other than the Bimba pneumatic cylinders. You could not, for example, make an air storage tank out of PVC or metal to use instead of the small tanks supplied in the KOP.
The reason should be obvious: anything that is pressurized will explode violently at some pressure, or if the robot is slammed hard enough.
If you are thinking of making an air-cannon (like a potato gun) you have another problem, I dont think you can get enough air pressure out of the compressor to launch more than one or two balls in two minutes this way.
You could use a pneumatic cylinder to push the balls out of your robot - almost the same idea, except the containment and release of air is controlled by the official Bimba cylinders. A 1" diameter cylinder with a 6 or 12" throw will take much less air volume per shot than a 4" diameter "potato-gun" that is 12 or 24" long. There is a bit of a trick you need to make this work - there are threads on this "pneumatic launchers", like this thread:
http://www.chiefdelphi.com/forums/sh...ight=pneumatic
Disclaimer: I dont speak for FIRST, but if I understand your intention correctly the answer is most likely no. Sorry I cant quote chapter and verse to back this up.