Import the skin/attackment and do a special effect on unit. Change "overhead" to "handleft" or "handright" and the model of the effect to the imported skin/attachment. This works but the units doesn't seem to grab the sword or what ever correctly. It like grabs it on the blade or on the barrel of a gun. Some skins dont work. I downloaded a bunch of guns and only one worked. The shotgun... Good luck with that
Create a custom item ability based on Item Orcish Battle Standard. Then change Art - Target to the model to display, and change Art - Target Attachment Point 1 to the attachment point to display the graphic (this will work for displaying non-weapon graphics as well). You will usually want the first item to be hand and the second to be right. Give this new ability to the item.