Mars could be given an atmosphere by dumping water on it, lots of water, enough to make seas. The gases for the atmosphere would be intro’d as soon as the water got there, or could be included in the chunks of ice delivered to make the seas. Between the gases and water vapor, Mars would have a nice, dense, temporary (centuries?) atmosphere that might support some plant life intro’d from Earth. Basically though, the only way we can live on Mars is to build indoor habitats, much of which would consist of inflatable greenhouses in which to raise sufficient/surplus food.