> for which gamepads do converters exist to plug them into superguns?
That is definitely the wrong way to ask. There is no standard for plugging external controllers into Superguns, nobody would mass produce such a converter.
You have to start with the Supergun and ask the opposite: Which Superguns come with gamepad inputs.
> does a Super Nintendo controller have one pin for each button?
I doubt it, but you can easily find out by looking at the pinout for a Super Nintendo controller. Google, then connect the dots.
And would it > be technically possible to create a cable with a Super Nintendo port on the one site > to plug a Super Nintendo controller into it and a supergun port on the other side to > plug the cable into a supergun?
Plug it in where? There is no "supergun port".
Yes, it's technically possible to connect any gamepad to an arcade PCB if you have the necessary skills and understanding of how the gamepad works.