It isn't necessary to make every world a duplicate of Earth. Variety is the spice, they say.
The good news is that you don't need to increase the mass of Mars. It's a nice thought, but who wants to wait around for that?
If you supplemented Mars' atmosphere up to about Earth pressure, it would take about a million years for it to leak away again. That's the good news.
The bad news is that the best gas to use for the purpose is carbon dioxide. So you still wouldn't be able to go shirtsleeve without a breathing supply and mask.
Planet Venus offers a plentiful supply of carbon dioxide, free for the taking, and most folks think that removing it would improve that planet too. Take all you want. If it leaks away, go back for more.
For me, it's not so important that humans be able to breathe the atmosphere of Mars without assistance; but I'd like to get it so that plants would feel at home there.
Imagine the size of trees grown in Martian soil and gravity!
And don't worry about a bigger Mars moon. We'll orbit it with a Rotavator. That should be exciting to watch!
You could start with carbon dioxide, seed the planet with plant life and let the plants convert the carbon dioxide.
But I don’t see how your going to get the carbon dioxide from venus to mars. Comets on the other hand can be gently nudged into trajectories that will colide with Mars.
As for venus, I say we engineer some life that can survive in the extreme conditions of Venus that will trap some of the sulfur. Turn it loose and see what happens.
Unless we can find some alkaline comets that we can crash into venus that will help trap sulfur.