One of my favorite ways to add attachments to units is to take an ability like item - armor bonus, copy it, set bonus armor to 0, and then set Art - Target to the attachment you want and the Art - attachment point to wherever you want the attachment to be (origin, chest, head, overhead, etc). Then all you have to do is add this ability to the units you want to have this attachment. You can either add them in the object editor or ingame with triggers. The best part is that armor bonus doesn't have an icon so it doesn't show up as an ability on the unit.