The part in the picture looks to me like a flow control valve. If the part is the same as sold by VEX, there would be no problem. Individual pneumatic parts are available from VEX Resellers (here is the flow control valve: http://www.robotmesh.com/vex-robotics/pneumatics/vex-pneumatics-flow-control-valve ) . The SMC part numbers are provided in the parts descriptions for the various components, so you can shop around to places like PLC Center or Motion Industries if you’re looking for a better deal, but you’ll find that the VEX prices are not dramatically inflated for these components.
To summarize the game manual, you can use as many pneumatic cylinders as you wish (in exchange for using 10 instead of 12 motors), but you are limited to only two reserviors (air tanks).