I love my big green egg. I have looked at the KJ and don't see why you couldn't use most any of the BGE accessories. The ceramic grill store is a great resource, I've bought several things from them.
One note, especially if you are looking at the bigger ones, when I looked at the big joe, there seemed to be much less headroom in the top of the cooker. The dome seemed flatter, giving less usable space. This was just by eyeball, and maybe I'm off base, but I'd recommend looking at these in person first.
And, I was not crazy about buying more made in China product.
Smokey Joe Silver