Even if this wasn't contained in the movie, you utilized a perfectly good technique that in symbolic logic is called the principle of addition. You simply bring in something else. As long as it doesn't contradict any of the givens, you don't affect the logic of the argument.
By this principle, you could have installed a tractor beam on the Mother Ship, given Major Tom psychic powers, and so on.
As contrasted to Dave's approach, changing the assumptions upon which the argument rests, which is what one comes to expect from a lawyer.
