Hi,
What do you mean by all pins are being used?
A 4x3 matrix would require 7 pins and I see 8 free pins (RC2-7, RA4-5, RB5) on the '4550 in your schematic...

If you put a slave PIC on the keypad you could program it to SEROUT/HSEROUT a byte each time a key is pressed and connect it to the USART of the 4550 (RC7 is the RX input for the USART and is currently not used in your schematic.)

/Henrik.