Indeed, this isn't D&D. Ask him first if he wants the 'frost hammer' because it would look cool, or for the extra damage. If it is for the extra damage, his best bet is to get a mage to cast a Cold Aura spell on him, or for a spirit with the Cold Aura power to possess him. In both cases, it is temporary, and in the second, he's no longer in control of his character. If he just wants it for the coolness, well, wait until 5E version of Arsenal comes out, and load up the Custom Look mod on it, to have an AR skin that has the head look frosty, with an icy mist falling off the head of the hammer, maybe with glowing Norse runes, as well.