if you dont mind hacking your pads apart its not hard to hook up quite a few to the keyboard port
but (if like me) you still want your pads to be usable on a megadrive then im as stuck as you, i only have 2 paralell ports and one of those is hooked up to a printer
i was thinking about using a usb to parallel adaptor but those only work on printers, which only leaves using a usb to serial adaptor and then using a serial to parallel adaptor, which isnt the most elegent of answers