Strange, normally people interface a standard keyboard to a microcontroller. You are doing the opposite.
Anyway, the simplest way is to simply implement a matrix keypad to the parallel port. If you want to interface to the PS/2 port, the protocol can be found at . The kbd clk and data lines are open collector. Just do a direct connection with a pull-up resistor.
Sincerely speaking, it'll be easier to just use a mini keyboard for your application. If you really need something really compact, I've seen keyboards that only contain the numeric keypad, though they are often a couple times the price of a full PS/2 keyboard.