So I've just checked and there is no way of me being able to target an item that is not in line of sight. I'm simply presented with what you see in the screen...an no entry sign and when clicking the building, there's no firing line - it can't be done.
So even though I can see the upper levels of a building (in truth, I can even see the lowest level...just not the base grid that it sits on and that the game uses to check for line of sight), I cannot suppress anything in that building unless my unit can see the base of the building.
It's very frustrating