ok, i dont know a proper command for this, but I know of one way to get him crouched - however I am not sure again whether he will be able to fire... anyway, when you have him in position, somewhere, somehow, you want a trigger to activate (you will have to work out how you want it to activate) then you can do this as the activation line:
pillboxman playmove "Combat to crouch"
where pillboxman is tho person you want in the pillbox ;D