To expand on what holmes4 suggested...
If you used all 18 inputs you could create a 9 by 9 matrix of buttons, or 81 total. That's not quite a full keyboard but fairly close. The code would be like to using a standard matrix keypad, the main differences being the array would be larger and you would have to be sure to maksing diodes for each of the buttons (specifically between the button and the connection to the pin for the row it's on).