I doubt you need to repurpose the fuel tank -- any rocket that travels to Mars would need to spend at least a few months in space to get there, so it will already need adequate oxygen tanks in its life support system.
And after you land on Mars, what are you going to fill that oxygen tank with? In order to have any use for it, you'd have to be producing oxygen on-site. But if you can reliably produce oxygen on-site, then you probably don't need storage for a huge extra supply for it. The existing life support tanks in the rocket's habitation module should already be more than adequate for storing enough oxygen to keep the crew alive.
(And this whole thing is assuming you'd land the entire spacecraft on the surface, which may or may not be the most efficient way to do things.)
Honestly, if you're serious about setting up a Martian colony, I think the best course is to first send robots to construct a base for you, so it's ready when you get there.

