Im a bit confused too.
So youre just running N64, and nothing else? Or youre running MAME and other emulators, as well as an N64?
If youre just using N64, dont bother with hooking up a control panel too it, N64 controller is the best thing to play N64 games. Using an arcade control panel setup is pretty annoying with N64, and unless you have the right kind of joysticks, that wont work right either.
If youre running MAME on a PC along with an N64 console, just use the N64 controller to play the N64, and use an IPac to hook up the buttons to your PC.
If you really do just have the N64 and thats it, and you want to use your control panel to play it.. uh, well.. it probably is possible to splice the N64 and hook it up, but that will require a bit of work to do.. I dont think you can just connect one wire to the other, youll have to hook it up with an HID or something.. idk.