The origin of your model has to be higher (relative to lower end of model) than the stair height to make the stair climable.

It will probably not look very good, but this can be adjusted by playing around with gravity values of the move routine.

Playing a different animation for staircases is not as eassy. You could however use a special stait case floor texture and then valuate its texture name using c_trace and then switch your animation code depending on the texture name. This is the easiest way I can think of...