>>5506754>increase the mass, volume and gravity to Earthlike levelsLet's do a napkin calculation.
Earth: 5.972 × 10^24 kg
Mars: 6.39 × 10^23 kg
Mars is about 10% of Earth's mass. It's probably much smaller than you thought because volumes aren't intuitive to people.
You'd need to add 90% of Earth's mass. This is about 5 x 10^24kg.
The main asteroid belt has about 3.3 x 10^21kg.
This means if you could spend the energy required to round up all the available mass in the asteroid belt it wouldn't even come close. So the answer is no. You can't.
