Yes, man can alter the Mars environment to make it suitable for human habititation. By using a more advance technology that can convert its soil particles (which is rich in carbon and sulfur) into greenhouse gases. The presence of greenhouse gases can intensify its thin atmosphere and heat up the dry frozen climate, thus producing water and making it suitable for plant life to produce oxygen. And when oxygen is present, only then, human can inhabit the planet.
Yes, man can indeed alter the mars' atmosphere to make it more habitable to human. But we can only attain it through the use of technology, advance technology to be exact. Maybe in the near future where there is enough instrument and research it would be really possible.  Mar's environment is really rigid, considering the thin atmosphere, lack of resources and minerals. Man need to do extensive researches on propagating an applicable atmosphere that can protect the mars' surface from solar winds and radiation from the sun. Results from researches implied that there is water within the surface so what we really need is the power of technology. Arigato! thats all