sort of... ufos wouldn't be coming from earth, necessarily, or even from off the planet. in raw game terms they simply "appear" and "disappear" and this could be interpreted as anything... descending into the atmosphere, launching from a hidden underwater facility, or teleporting from another dimension. i'm thinking invasion of mars, with VERY long delivery times for personnel and equipment and high shipping costs, and very little return on selling things. the alien missions would represent a "defensive" shift in the alien strategy, and alien base attacks would have to be somehow made more likely. i think putting retaliation missions on the schedule would be able to achieve this in the short term.
of course, what you said is viable, too.