Actually, that's the way I think we need to terraform Mars. If neither it nor Europa turn out to have life, slam the latter into the former at an oblique angle and it should add water, re-liquify the planet, and add radioactives to the core. If you to it at just the right angle, you might even get the large moon. It's strongly advised that we really know what we are doing before moving an object as large as Europa around the inner solar system since any significant interaction with Earth or a collision could be catastrophic. And, yes, I do think that it's possible to destabilize the orbit of Europa and send it toward Mars. (Why, yes, I did read Larry Niven's
A World Out of Time.)