Dear @Xing.Tan,
Taking the absolute value of the hydrostatic stiffness does not make sense to me. Moreover, you are missing the contribution of full-system weight to the restoring (stiffness) in the pitch and roll directions (that is, mass * Gravity * CoG). I would assume that if you add this gravitational stiffness to K_moor + K_hs, you’ll get K_OpenFAST.
Regarding your second question, what extra stiffness and damping are you referring? Regardless, if you have an external stiffness and damping that you want to apply to a HydroDyn module that only has a the strip-theory solution enabled (without potential flow enabled), you could add a “dummy” potential flow solution with PotMod = RdtnMod = 0, which would then allow you to set nonzero values for the platform additional stiffness (AddCLin) and damping (AddBLin).
Best regards,