Will snow and ice buildup around the outside of Heat Pump damage it?

I've never heard of one getting "damaged" from anything like snow and ice buildup, but you certainly shouldn't run one when it's covered in snow and ice. It's possible that the lubricant in the refrigerant could freeze over and stop lubricating the compressor, but I've never heard of such a situation. The high limit pressure switch for the refrigerant pressure line should pop before that happens, rendering your unit useless until it is manually reset or temperatures rise. 99% of the units I work on have outdoor thermostats that stop the operation of the unit completely when temperatures get unfavorable for their efficient operation (it becomes more efficient to run backup heat).