No, there is no way to teach Zap Cannon to Mewtwo. But it's a bad move anyway since it only has 50% accuracy. A wifi event just ended for a Mewtwo with Electro Ball, so if you ask on other forums or on our chat someone may be able to trade you one.
Besides that, Mewtwo learns Thunder and Thunderbolt by TM, both of which are great moves.
