I don't know if you can force a unit to crouch, but I do know that you can force a unit to stay standing or prone. If either of these will also suit your needs, the command for that is as follows:
; Force the unit to stay prone.
soldierOne setUnitPos "Down"
; Force the unit to stay standing.
soldierOne setUnitPos "Up"
; Let the unit decide as the situation dictates.
soldierOne setUnitPos "Auto"