If you're willing to launch a Shuttle without any payload, you could put a couple tanks in the cargo pay to carry hypergolic propellants for the OMS engines. Subtract the tank and plumbing weight and you might be able to carry as much as 40,000 pounds or so of propellant.

I seem to remember reading that the Shuttle launches with about 20,000 pounds of OMS propellant, which is sufficient for about 1000 feet per second of total Delta V. If that's accurate, then 40,000 pounds of additional propellant should be sufficient for another 2,000 feet per second or so of Delta V. That would be enough to boost the Shuttle's orbit quite a bit but probably not enough to be worth all the expense.