You need to find a wiring diagram for the keypad - what are the 8 wires connect to?
You can try and use a meter to find out which pair of wires are closed (and the rest are open) for every key press. That is you need to test 7+6+5+4+3+2+1 combinations whilst holding a key down. .. if you think about t long enough, you can write an sketch that will do that for you 
Or search. I am sure someone already has done this (I did once for some other special switch... cant find the code, otherwise I would have attached it)