Are you thinking a "Touchpad" or an analog "nub" (The kind in the middle of the keys that you just maneuver with slight pressure)...
In my opinion either one would not render itself well to playing games with... With a touchpad you would never be able to "spin" the ball... And one of those "nubs" would just drive me crazy trying to play Marble Madness or other such games...
If you are talking about the touchpad, they have those available for Desktop computers... They usually connect through the serial port. There's an electronics store close to me that carries them for less than $20 I believe. I'm not sure how easy it would be to do something with one of those "nubs" though...
In any case, if you decide to mess with this idea further, please let us know how you like it.
~ tmasman