You can add more than two joysticks. I spoke to JibbSmart the other week and he's done it for KarBoom. He uses the functions from windows.h that comes with the engine. Maybe try that out?
And I also advice you to stick with split-screen. Network is a whole new deal and porting a single-PC-game to being a network game takes years.
~"I never let school interfere with my education"~ -Mark Twain