Need help - non combat unit with bombard attack

Ok, so I'm trying to make a new type of unit - a 'support class' that can stack with regular units. I'm trying to make artillery units be able to act like civilians, where one is able to be on the same tile as a military unit. I can make a unit stackable, but it can't bombard, or I can make it bombard but it can't stack. Is this something you can do through lua, xml, or sql, or does it have to be C++?

Thanks!