If you make it fast enough it will stick around for a while. "A while" in planetary terms can be a few hundred thousand or million years. So it's possible you could produce sufficient atmosphere to make it breathable, and it would remain so for longer than human civilization up to now. Of course, by possible, I mean with the right tools and the resources to support that, which would be substantial. Feel free to find out how many comets you would have to impact into Mars to get that.