I cleaned my Tippmann 98' Custom a couple days ago and looked at the gas management system in a lot more detail than I normally would. If im not mistaken, it would be EXTREMELY easy to modify! You can get these guns fairly cheap especially on Ebay or something. I found the tear down manual in PDF form online. This PDF doc is the exact booklet that comes with the gun. The booklet right now is stored in the gun bag along with the one for my Spyder.
At first glance of the tear down manual, its a bit overwhelming. Theres a LOT of stuff going on. Look on page 5 for the tear down manual. The PDF may take a minute or two to load fully so wait a little bit.http://playpaintball.com.ua/load/98cmanual.pdf
If you look in the center of the page, you will see "Valve" and to the left you will see "Power tube". Youll need that whole assembly there. Youll also need the "gas line" and the "tank adapter". After that you should be good to go. Theres a small pin sort of thing that protrudes through the hole in the back of the Valve and when that pin is pressed in, the air flows. I havnt hooked up the tank while the guns opened up because thats dangerous and such... plus I dont want to risk damage it loll
Hope this helps