I've never done it, but I've heard of guys filling the stock tank with boiling water to soften it and then using compressed air to expand the tank. It gets you a little more capacity. I've also seen auxiliary tanks that either fit behind frt or side # plate or attach to frame.
Actually we want the extra capacity for GNCC races and some Harescrambles that go past the hour mark.
I did the hot water thing and got almost 2 pints more.... It just shrinks back some since the plastic has a memory. I guess I can do it once a day for a week and try to get it to stay...