You add the cable capacitance to the capacitance of the RIAA unit. So, if each has 200pf, your total net capacitance the cartridge sees will be 400 pf.
Cheers, Crazy Bill
PS. It's usually best to get the lowest cable capacitance and adjust the RIAA preamp. If your cable capacitance is too high, you lose a lot of adjustment that you might need.
Don't be afraid to experiment. The manufacturers recommended capacitance might not be the best fit for YOUR system. Play around and trust your ears