Dear @TianYuan.Wang,
More often than not, the additional platform stiffness and damping are calibrated to known solutions rather than “calculated”; if you want to use equations involving 0.5Cdrho*Area I would expect that it would be easier to use the strip theory (Morison equation) formulation directly.
That said, if you know that draft force is applied at a given point that is offset from the platform reference point, you could calculate the equivalent AddBQuad matrix by a suitable transformation of the kinematics–that is express the motion at the given point based on rigid-body kinematics of the platform reference point (forces transfer directly with moments added due to the moment arm).
Regarding transferring matrices expressed relative to the platform reference point and another point (such as CoG), please see the following forum topic: Time domain analysis by AQWA.
Best regards,