well for a 4x3 keypad you would need 7 inputs according to Nigels toturials. that would be a problem if you have only 14 pins, im guessing u have got about 5 inputs right? i cant see your drawing well so i dont know. i dunno if its possible but u might be able to use the analoge input of the IC (if it has one) to tell the difference between the buttons pressed. if not then i dont know sorry
Replace the 100R resistors with 1N4148 diodes pointing down.
Replace the switches with your 4*3 keypad.
The 4 row connectors go to the diodes.
The 3 column connectors go to 3 I/O pins.
In your initialisation software,
Initialise the 3 pins connected to the keypad to outputs and set them high.
To read the keypad,
Set the 4 data lines to input.
Set one of the 3 pins low.
Read the 4 data lines.
The value read is the column connected to the pin you set low. A zero = key pressed.
Set the pin high again.
Repeat for the other columns.
Set the data lines back to output.
By my counting, thats a total of 9 I/O pins needed.
This will also take you (eventually) to a Microchip site with tricks and tips for the PIC you may find quite useful.
Finding a commercial keypad to do this will probably prove impossible, so you may have to roll your own with 12 (16 if you want) switches on a proto board and an interconnectiong cable.
With a number of carefully selected resistors, you can make a unique voltage divider for each button on the keypad. You can then sample the voltage created using a single analog input pin and the A/D converter.
Here is an example with 8 discrete switches. You will have to adjust it to work with a matrix style keypad.
There IS a way to wire a matrix keyboard so that every key gives a unique resistance, I'm fairly sure it's been posted on these forums previously?. I remember thinking it was rather clever!.
As long as you have a spare analogue input (or don't mind doing software A2D) you only need the one input.