Theoretically they can split the water/ice jnto oxygen and hydrogen. A fuel, a catalyst, and breathable air to mix with nitrogen.
I think nitrogen would be the key missing ingredient. Although I did just find an article where they have discovered nitrogen in the soil - amounts of 1,000 parts per million. Released upon heating. Nitrogen was created they think from meteorite strikes and lightening. If that is the only source, I’m not sure we could produce enough on Mars. Our atmosphere is something like 80% nitrogen.