AD5933 Feedback and Calibration Resistance
\$R_{FB}\$ is part of a current to voltage amplifier. \$R_{FB}\$ must be selected to keep the signal below \$V_{DD}\$. Replacing it by a capacitor will change the function of the amplifier, so there is no assurance that the circuit will work as expected. This will also remove the DC part of the signal, and the IC probably needs it since it specify a maximum for the measurable impedance.

What you can do is calibrate with \$Z_{ω}=R_{CAL}\$ then swap it with a capacitor. The part have a maximum impedance limit so you may have to put that capacitor in parallel with a known resistor. If you are only interested in the chnage in capacitance rather than the absolute value, you could make a RLC resonator with the varying C, measure its resonance frequency with that IC, and forget about callibration.