Dude, don't do it! JK

Actually I used a Micro Innovations USB to PS2 Mouse & Keyboard Adapter on my 1st mouse hack.
It worked flawlessly with the mouse however the keybaord response sucked (shouldn't be an issue if you are not gonna use that option).
But before you do that might I suggest trying a search for a Kensington ValuMouse Three button USB.
These work great for Trackball & spinner hacks.
The PCB is very easy to work with & has a small footprint.
Good luck!Z