Even if you could add to the mass of Mars, you would still have to supply gases to give it more atmosphere. If you could take the water ice from Ceres (it has more fresh water than Earth does) and drop it on Mars, then separate out the oxygen, that would help some. Future colonies on Mars will probably be underground.
Probably Ceres itself will be a future colony for spacemen.
Over a few centuries we could terraform Venus to give it a breathable atmosphere but it is still too near the Sun so would be too hot for us.
Atmosphere is secondary to a moagnetic core.