First: Could we populate another planet with genetically modified organisms? Second: Should we?
Dirk Schulze-Makuch, Professor of Planetary Habitability and Astrobiology at Technical University Berlin, President of the German Astrobiology Society, and Co-author of The Cosmic Zoo: Complex Life on Many Worlds:
I don’t think we’re there yet, in two senses. We don’t know the environmental conditions of other planets well enough, and we don’t know how to optimally tune the genetic code of an organism to thrive in that extraterrestrial environment. The only planet where I see this as a possibility in the near future is Mars, which we know best of all the planets and moons in our Solar System.
But even if we can do it, I don’t think we should. It would be a very human-centric approach. Instead, we should try to explore the diversity of life that may exist on other planetary targets. In regard to Mars, that would mean exploring whether indigenous (microbial) life exists, and if so, studying how it is different from life on Earth. (Even if there is a common origin, evolution in the different planetary environments would still have resulted in significant organismic changes.)
If we encounter a habitable planet—and one which we know for sure is uninhabited—the question becomes harder to answer. We can come to that when the situation arises—which it won’t for a very long time.