I'm guessing that the ribbon cable you're describing goes from the circuit board to the console plug. You can't solder buttons onto there, that's a digital signal, not just button inputs.
And as for hacking the whole Dual Shock 2, I doubt that you can. The pressure-based analog button style is such that you'd have to solder to points away from the pad that is under the button, in another section of the circuit. The other wires are quite small, too, so just find a PS1 controller to hack.
If you don't even understand this lingo, you should not at all attempt to hack a PS2 controller. Hack a PS1 controller. Find a PS1 controller that someone ELSE has documented if you are REALLY unsure.