It's most likely possible, but I don't know if there's any documentation on how to do it yet.
You will need to hack the character save files, and the following knowledge will be needed:
a) which bytes to alter (this can probably be determined by a detailed comparison of NPCs' vs. PCs' saved character files - there is presumably an index in the NPCs' save files that tells them to use a given monster/NPC combat icon index instead of PC-style combat icon data. That this sort of data structure must be true is even more evident to me from the fact that I recall you can in fact alter NPCs' combat icons in the camp menu to use PC style icons - however, you cannot revert this change once you do.)
b) a list of what combat icon indices correspond to what actual icons.
