[nbadg]

Atmosphere stripping due to solar wind is a natural process that takes millions of years. Atmospheric production is a deliberate process limited only by available energy and raw materials. And, of course, the economics. But the point remains: you absolutely can densify Mars' atmosphere, and you won't have to worry much about it until you get to the point where you start running out of raw material to replace the very slow loss.

My favorite idea to re-liquify the Martian core is to move Ceres into orbit around Mars. It'd take a very long time to do it, but you've got millions of years. But the point is, in the meantime, having a dense enough Martian atmosphere to walk around with just a gas mask is certainly attainable in a lifetime, so long as you have enough energy production to do it.

[lxmorj]

It might not take that long given the right tech. You'd need a legit fusion reactor, and enough material to build an long mass ejector and start shooting 0.01c ice pellets into the outer solar system, driving the rest of Ceres toward Mars. The same mass ejectors could be tuned to bombard Mars with ice at a much lower speed later, if needed for terraforming.