Lego WFC, brilliant!
Nice work!
do you mean an external cap in series? if so...
I don't think you can use an external cap to make the wfc resonate properly, the reason is that you have two resonant charging chokes, each resonanting with one side of the cell, or it is resonanting on/from both sides of the cell at the same time, as well as you need equal and opposite voltage on each plate, so if you add a cap and use the wfc as a resistor then it will change the point at which you are resonating about, and have different voltage on each plate.
if you used two identical caps on each side of the wfc in series with the chokes then you could get it to resonate on both sides with that
or if you used your external cap in parallel with the cell that might work