For this mod i will make my shoulder button combination will be UP and B because this is how you do the special attack in two of my favorite games (Castlevania and Ninja Gaiden), it can be changed to any combination of buttons to suit your favorite game though.
Step 1: What You Need,
Prototyping board (I used strip board)
Button (get a decent sized one)
2 PNP transistors (NPN could be used but you would have to change the wiring a little, I will be teaching you how to do it with PNP)
Step 2: Making the Board
Looking at the transistors with the flat part towards you put them in the board so that the middle leg from each will be connected to each other and the right leg from each will be connected to each other. The left leg will be on its own row.
Now solder them in and make sure that you can bend the transistors flat and that the mounds of solder you get are as small as possible, there is very little room inside the controller and i ended up filing down the parts i soldered to get it to fit in.
Note: you could solder the transistors directly to each other and save some room, i didn't do this, i only thought of this after.
Step 3: Add Wires
Step 4: Add the Button
Step 5: Open Up the Controller and Connecting the Wires
Looking at the green side of the circuit board there are several black circles, these are the contacts for the buttons, and two rows of pins for the microchip.
On the right hand side there will be two little copper circles, the left one is ground.
solder the wires that come from the button and the right leg of your transistors to this point.
Find where the contacts of the first button you want to use in your special attack button and follow them until they end on the end on the chip in the middle of the board.
Take the wire for the left leg of one of your transistors and solder it to this point.
Do the same thing for the second button you want in your special attack button.
I Have marked where to solder the wires for the UP and B combo.
Step 6: Test
Make sure all the normal buttons work as normal and then test the special attack button. If anything doesn't work right then go back and double check everything.
Step 7: Drill the Hole for the Button
Step 8: Put Everything Back Together
I didn't have to use any glue to secure my button in place, it just sits on top of the original circuit board and that holds it in place nicely. You will probably have to glue it though, or if its too big cut away a part of the original board to allow it to fit (make sure you don't break any of the connections if you do this)
Put the back cover on and replace all the screws and you're done!