Where can I find PCB templates for add on boards
When I was designing something similar for MSP430 development boards, I just used a vernier caliper to take measurements, noted them on a piece of paper, then designed my own board.
I'd recommend doing that, as this way, you get more flexibility over how you want your board to look. Just make sure you are careful with the measurements.
Here is an example of the most recent one. I've done a couple now for different boards and as long as you take care, measure 4 or 5 times, then you should be fine. I've never had an issue
 Here is it attached to the development board
 Here it is in full, customised to my own needs and shape