Though I did figure out a way for NPCs to use stances. You would need a set of FRMs where the critter is standing and laying down. At the start of combat the critter would drop to the ground and use the prone set of FRMs.
It needs to be scripted, I'm not sure how to script it but I know it can be done. In FO2 Goris used 2 different FRM sets, one with his robe on and the other with his robe off. At the start of combat it would go through the cycle of changing FRMs, so you could just replicate that process but instead of using cloak/no cloak you have stand/crouch
It would be a good visual surprise for someone watching to see the bunch of npcs suddenly hit the deck.