Hey, havnt tried this myself, but from what I can gather, if you
setBehaviour "CARELESS"
then units wont react to firing, or anything else. So if you do that, then, as dan ick suggested
setunitpos "up";this switchmove "combattocrouch"
they should not react to firing, and stay crouched. However, once you want them to react and shoot, you will have to set their behaviour back to COMBAT or whatever, and not CARELESS. - once that happens, I dont think you will be able to get them to stay crouched.
Actually, when I think of this it may not work, but give it a try if you like...