Is it possible, or could it eventually be possible, to alter the atmosphere on a planet to suit us?
Of course it is possible. We’re altering Earth’s atmosphere. The problem is the amount of time it takes unless the planet already has an atmosphere closely similar to ours. As for Mars, it ain’t gonna happen. Mars does not have a magnetic field to deflect solar winds and radiation, so no matter what we try in its very thin atmosphere will get stripped away quickly.
I may have heard a SETI person (possibly Seth Shostak) state that if we find a planet that is nearly ideal for us to live on we may eventually have the technology to alter its atmosphere.
As much as I appreciate what Seth is doing with SETI, he’s a dreamer when it comes to humans inhabiting other planets. Identifying a suitable planet may be within our technological ability in the future, but getting there is another matter. Lawrence Krauss addressed this in one of his books about the physics in science fiction. Accelerating a school bus size craft to 0.25C would take more energy than exists in our solar system, at the very least. Even traveling at 0.25C it will take decades to get to any other star system, and we’ll have to send robotic probes ahead to make sure we find a suitable planet. Then we have to alter the atmosphere for our biology. By the time that is done we’ll either be extinct or not need to travel to another solar system because of technological advances or societal collapse.