Hook up (wire) the varactor properly with all needed grounding and biasing sources, to facilitate measuring of its input impedance with a current ix. Give the Vbias voltage source a variable name of "Vbias", do ac analysis, instead of sweeping frequency, sweep Vbias, then use your calculator:
Cap = 1/Impedance(f0)/(2*pi*f0)
There may be a direct way of plotting Cgg capacitance, but I kind of forget without looking at Cadence right now.
Yes, if your concern is that after adding Vbias, you short out the ix, you can put a large (dummy) inductor (say 100 H) in series with the Vbias.