I really don't think it would be John. For one, he couldn'tdo it himself, we've seen inside him enough to know he's no practitioner and even if he wanted to bring in a heavy, he wouldn't want something so dangerous to "his" town/universe.
And two, I'm pretty sure his contract with Monoc Securities would preclude them summoning of Outsiders on his behalf, it might even nullify his contract if he tried. Wasn't, Odin and company one of the previous forces defending the Gates?