I just bought an xterra and did not realize that with all my driving i wold have to fill up so often. I didn't study enough.
Anyway, does anyone know of a company that sells extra capacity fuel tanks that would fit an '02 xterra?

In my opinion that's a custom built unit.

No such thing, and there are only a handful of people that find filling up an extra time a week to be such a burden that they would even consider the cost and effort of a replacement tank, therefore there will never be such an offering.

Wouldn't hauling around more gas just further reduce the pathetic gas mileage, making you visit gas stations even more, which you're already trying to avoid?

I gas up every 5-7 days.