An ABM can't be somehow "retooled" to make an offensive weapon. The notion is silly, we have intermediate range ballistic missiles that we can build and deploy as needed, no need to somehow jury rig an ABM. An ABM is designed with short range and very fast acceleration for obvious reasons. The rocket motor is specific for that purpose. And of course the warhead is designed to intercept an incoming unarmored missile, some are designed to try and hit a bullet with a bullet.