Depends on the level of integration and how the reader passes data.
Some readers will pass pin data as weigand signals, so you could, in theory, put in a site code and credential id and then translate via Elk's calculator and have it in there, but basically you're going to be looking at a lot of keypresses for little return. If you get away from the site ID and the 5 digit credential idea and set the unit to blast a basic 4 or 6 digit pin and kill the preceeding site code or narrow it down to a 4 digit code with preceeding zeros, you could use the PIN portion.
In the case of a prox/pin combination, the same would hold true, however you would need to figure out on the backside how to work the logic. Unfortunately the M1 is not a full blown access system.
