and staying indoors.
If we added Mercury (which is denser than Mars but smaller obviously) to Mars, the little virtual calculator gizmo I've got here puts the combined mass at 9.7219E23, which is 16 per cent of the Earth's. Venus' mass is 81 per cent that of Earth. There really isn't any way to find enough material to make Mars like the Earth.
If there were lots of water available to bring Mars' mass to that of the Earth (and there could be), Mars could be converted to a planet with a planetary ocean, Earthlike atmosphere (synthesized from frozen gases imported from the outer Solar System, which is where the water could be lurking), and floating cities for humans. The final diameter of water-covered Mars would be larger than that of Earth, because of lower density.
Current Mars is just under 11 per cent the mass of Earth. Moving nearly 90 per cent of the Earth's mass is well in the future. IOW, the only practical prospect for the foreseeable is to build habitrail-style habitats on Earth (could be just inflatable structures) and deploy them on Mars, inflating them with the Earthlike atmospheric mixture, and staying indoors. :')
and staying indoors.