from what I understand, the i-pac is an encoder/decoder for the buttons and tells each button what to do. If you hack a joypad, that encoder/decoder will already be in place, right.
The hacked pads I have seen, hook directly to the buttons, if I am not mistaken. An for what it is worth, I am not an expert and have never hacked a pad, use the search option and type in hacked______ what ever, you should be able to get some post on this topic.
Good luck and post what you find. I will do the same!