Standing and aiming:
this setBehaviour "COMBAT"; this setUnitPos "UP";
Crouched and aiming:
this setBehaviour "COMBAT"; this setUnitPos "UP"; this switchMove "Crouch";
Prone and aiming:
this setUnitPos "DOWN";
(The soldier looks like he is looking through the sights while he is prone by default.)
Remember if you want the soldier to behave normally afterwards, make him setUnitPos "AUTO";