You cant make him crouch using just the editor on its own.
You could put this in a trigger:
Soldier1 switchmove "Crouch"
But this only fires every 0.5 seconds, which is not enough as he just bobs up and down.
You can do it via the editor, but you will need an addon.
Or you will have to do it via an external looping script, there are plenty of examples here.