Not talking about connecting a genesis pad, but more of a crazy idea that I unfortunately have no skills or tools to pull out myself.
It would connect to BOTH left and right joystick ports (meaning it would be only suitable for single player games), the left joystick's, well, joystick would be mapped to a dpad, while the right joystick would have its direction mapped to 4 buttons (let's call them a, b, x, y) in a standard configuration, while fire buttons of either would be mapped to L/R on the pad. Overall, the entire contraption would be reminiscent of a SNES gamepad, only without start/select buttons and with two cables sticking out of it.