It sounds like impossible, but I find a way to do it.
Firstly, AI soldiers can attach fixed weapon only when they run to it. In other words, you must set a waypoint marker on the weapon, give AI a waypoint list, then change the soldiers alarm state to red. If he/she just stand behind the weapon or slow walk to it, nothing will happen.
Secondly, AI soldiers must have a weapon. Of course, they will use their own weapons to fire. But the fixed weapon will fire simultaneously!
I also upload a tutorial and example maps about this discovery. Wait someone to check it.