The only thing you need to do with a store bought pre-cooked ham is to pull off the skin. Use your hands, it comes off relatively easily, or cut it off with a knife.
Now get a sharp knife and cut slices in the ham from left to right. Cut as thin or as thick slices as you like. Just like a hasselback potato you don't want to cut all the way through. I typically stop 1.5" from the bottom.
Now sprinkle all the sumac in between each slice, and if you have any left, sprinkle the rest on top.
Now cover the outside of the ham with the bbq rub.
Fire up your grill and prepare it for indirect grilling at around 120° C / 260° F. Once it is up and running you place the meat on the grates. Place a few wood chunks on the glowing embers for smoke flavor.
After 45-50 minutes it is time to apply the first layer of syrup glaze. Keep adding a layer of glaze every 15 minutes until the ham is done.
The ham is done when internal temp reaches 50-55° C / 122-131° F.
Bring it in and slice it all the way through. I built a foot long hero with ham, lettuce, paprika relish and other good stuff.