you'll have to rip apart the pad.... but if you do a good job... you will be able to still use the pad, and the extra wire will connect to the arcade control.... (so, you don't lose the pad...)
I think Dave_K is a guy who did this with an xbox control... (I'm not sure if he is the one... if its someone else... I'm sorry for bring the wrong name...)
but he inspired me and I did the same.....
hacked 2 xbox pad so that the pad is still useable and it can connect to arcade control....
ps: ps2 original pad is very hard to hack.... if you do want to hack, use madcatz pad.... much easier.... (and they use common ground for the 2 analog triggers...)
hope it helps....