It sort of depends on your definition of terra forming. Like if your goal in Earth 2.0 … Then your likely not going to get there… there not enough free mass in the solar system to build mars up to size of earth. But if your goal is to make mars livable. we could build into mars crust to allow for high pressure areas … If the goal is to get a temporary atmosphere … that would take a few millennia active effort
Should we terraform Mars? (en.wikipedia.org)
If so, this should not preclude us from cleaning up our own planet first!