This isn't the answer you want but, you could always disassemble your game pad, and disassemble a keypad and a joystick, and solder your gamepad contacts to the keypad and joystick contacts, and viola!
or you can take extension cables (if they exist for the N64) and hack together your own cable.
They do exist. Though you would have to look hard nowadays since N64 is no longer being made. So naturally, the third-party products faded away over the years. I used to have an extension cord for N64.