Many people have written about ways to hack a mouse to a trackball, so I decided to give it a shot. It was trickier that I first though it would be, as it seems some mice are configured differently from others, but I did finally get it to work very well. Sean has some great info on wiring up a mouse on his page, and is recommended reading.  I took it a step further and wired both the emitters and receivers from the mouse to the trackball.  This way I didn't have to wire the trackball to a separate +5V power supply.