They don't output different voltages. I know because I made myself a NES - USB converter. It is all just a shift register. You clock the chip inside the controller and it outputs the information about which button is pressed.
It's a very simple system and with some clever coding you could probably achieve it easily enough. If not, there's a company called ZeroSoft which gives you a schematic and code to make your own NES - USB converter. You can turn it into a mouse or you can use it as a joystick. I think the newer versions let you do both if you do a special key combination.