Maybe something else could be used like send a serial signal from the keypad. Did a quick search and found this.
http://www.rentron.com/Ser-Key.htm
Its not as simple as using only resistors but it would probably be more reliable and easier to code for. Or if not serial then maybe a morse code like signal could be sent from the keypad. Overall the circuit would be more complex but it would use less pins on the Arduino. Cheers!