Let's say you are a scientist in the future trying to save the world that started to crumble down because our atmosphere is slowly vanishing. To save the world you have to make a "new atmosphere". Can it be done? And if it can, how?
It depends on what rate the atmosphere is depleting. This depends on how. If a solar flare is destroying the atmosphere, it should be some time before it is gone. Think millions of years. If it is something man-made, then we should start by stoping whatever we are doing that is depleting the atmosphere.
If we have achieved easy space travel (through the solar system, maybe not interstellar) we could have shipments of gasses from the gas giants to add to the atmosphere. Yes, it could in theory be that simple. In practice it would be very difficult, if not impossible.
If the atmosphere should be gone in a single lifetime (and that is an unrealistically fast pace) then there is no way to replenish it. We would have to totally reverse whatever natural or man-made processes that caused the disaster. That would likely be the equivalent of trying to reverse global warming, or make a hurricane turn tail and head in the opposite direction.