This is actually pretty easy. You'll need to crack open the case and drill a hole for the tactile switch, we use I think tac switches that are 3mm across on the face but I wouldn't swear it, I think that translates to a 9/64" drill bit, then we use hot glue to glue it in place. If you don't already have some tac switches you may be able to get them cheap (shipping will cost more than the switches lol) from
http://www.sparkfun.com. Mouser.com and digikey.com carry them but they may have minimum order requirements.
Hint, you can use a toothpick to help hold the tac switch in place while gluing, I press against the backside of the tac switch with a toothpick and work my way around the tac switch with the hot glue gun. Don't squeeze glue underneath the tac switch (like between the tac switch and the shell) it could stop it from moving (don't worry if you mess it up hot glue is relatively easy to scrape off with xacto knife). I just run some glue basically around the edge of the tac switch, doesn't take much to hold it into place. Be sure to click the tac switch a couple of time (while holding it in place with the toothpick) to make sure the action is working and hasn't jammed up. You still have a chance to move the tac switch around just a little bit while the glue is cooling down, to make sure it clicks/presses nicely.
Once you've got the tac switch glued into your shell where you want it, you need to solder two wires onto the tactile switch. One side of the tac switch needs soldered to ground (any ground will do, if you don't want to look for one, use the 'G' connection in the attached picture). To the other side of the tac switch, attach a wire to the 'H' point seen in the attached pic. A digital multimeter will help you figure out how your tac switch works, the tac switch probably has four legs on it, but you only need two of the legs - finding the right legs is really a matter of using a multimeter. The tactile switch should be normally open - momentary on type. So when you click it, it is essentially shorting out the G and the H points together (simulating a press of the left thumbstick).
Now when you tap the tac switch, it should do the same thing as clicking the left analog stick.
My wrists kill me after clicking the sticks, so I'm with you, man!
E
. I'll let you know how it turns out when I can find the time to mod it. Most of my time is being consumed by an attempt to turn a toaster into a computer for one of my classes. Yes, a toaster.