I don't think they make any. When the ball comes out, it isn't traveling like a regular ball, so an extension would probably only cause breaks.
-Tippmann Model 98 (with Flatline, stock, bipod, Dye Sticky grips, Psycho Ballistics cocking knob, remote, regulator, expansion chamber)
-Tippmann 98 Custom (with ACT, response trigger, BT Sealed Bolt System, Allen stainless steel accents, J&J 14-Inch Ceramic barrel, Dye Sticky 3 grips)
-PT Xtreme JR (with extra magazine, Ronin holster)