It doesn't matter whether the capacitor happens to be charged or not. It can be charged to 1/2 volt, or it can be charged to 1 thousand volts, or it can be totally uncharged and wrapped in tissue in a box on the shelf. None of that has any effect on its capacitance. Whatever capacitance it has, that's what it has.
In order to increase its capacitance, you would have to . . .
-- increase the area of the plates,
or
-- move the plates closer together,
or
-- fill the space between the plates with a substance that has a greater dielectric constant than air has.
