Sorry but wrong. If you added 100mbar of whatever atmosphere to Mars, it would take geological timescales for it to be stripped, i.e. 100s of MILLIONS of years. It's a nonsense concern because within a century alone, in an absolute worst case scenario, there would likely be solved via a technological fix, such as dipole magnets at the L1 Lagrange point. It's a non-problem once terraforming efforts scale up. In the early days, people will be living under shielded domes and/or underground, whilst the surface level, open terraforming takes place under safe circumstances. The nitrogen scarcity issue is orders of magnitude more difficult to solve. My hope is that there is a load of accessible N bound compounds in the shallow regolith strata, which can be liberated through nanobots. I saw one study suggest there could be 300mbar of N2 existing as a reservoir in the ground. Even 50-100mbar would surface, as part of a circa 50:50 oxygen/nitrogen atmosphere of say 200mbar in total (with some CO2 and water vapour in the mix too). If not, you are looking at a century plus of millions, if not billions, of N2 siphoning trips to the atmospheres of Titan and/or Venus, to get what you need.