Although the code for the VA calculations is open, you cannot modify VA directly. You have a few different options to do this calculation:
1) Simply copy that code, modify it to do the calculation you want, and run it from the python workspace. The modified code might look something like:
t68 = data.timeSeries('Final Pb206/U238 age').data()
t75 = data.timeSeries('Final Pb207/U235 age').data()
disc = 100 * (1 - (t68/t75))
data.createTimeSeries("% Discordance", data.Output, None, disc)
2) It is a simple enough calculation that you could probably set it up in Tools->Calculator. Just keep in mind that the spaces and / in the names are translated to '_' in the expression, so it would be something like:
%Disc = 100*(1 - Final_Pb206_U238_age/Final_Pb207_U235_age)
3) Wrap up the calculation into a simple UI plugin that you could invoke from the menu. Something like this example, but you would have the action call your discordance function when it is triggered.
And probably more!
All the best,