The mass of the plant would need to increase.
We could easily ship all kinds of matter from earth to accomplish that.
It is said the weight of the earth keep growing 3 or 6 billion tons per year.
Now if we could turn that around in someway and send that weight to mars then the larger the mass then the bigger the gravity and the larger/better atmosphere the mars planet would be.
The mass of Mars is 6.4169 x 10^23 kilograms. That's
641,690,000,000,000,000,000,000 kilograms. Your 3 to 6 billion tons per year is
000,000,000,002,721,554,220,000 kilograms*. Note all those leading zeros.
If we got that 3 to 6 billion tons per year to Mars, it will only take about 3,000,000,000 years before the mass of Mars goes up to 6.5169 x 10^23 kilograms. That's 3 BILLION years.
Now, suppose you wanted to launch enough matter from the Earth to raise the mass of Mars from 6.4169 to 6.5169 x 10^23 kilograms. Not enough to make any significant difference in gravity, but I think you'd call it a start. Quick back of the napkin calculation, using the escape velocity at the surface of the Earth (11.2km/s = 11200 meters per second), and 1/2 mv^2, gives 6.372*10^7 Joules per kilogram. The change in mass is 0.1 x 10^23 kilograms, giving us 6.372 x 10^29 Joules of energy. How much energy is that? Rough estimate of all remaining fossil fuels on Earth, plus all nuclear (fission) fuels (e.g., uranium) is less than 10 petajoules. That's 10^25 joules. So, you need 10,000 times more energy to do that than all the fossil fuels and nuclear fuels that exist on this planet.
So, no. You're not going to do it.
*(Depending on which definition of ton you use.)
**I did the calculations quickly & without much thought on the back of a Tractor Supply receipt in a dimly lit room. Please correct any inaccurate calculations.