Dear @Emanuel.Rergis,
The stiffness matrix that has been prescribed for the OC3-monopile is included in the sample UserPtfmLd_CS.f90 file shared through the the following forum topic: Turbine-soil interaction. Influence on the mode shapes. - #13 by Jason.Jonkman. Note that stiffness values are only provided for surge, sway, roll, pitch and their couplings. As prescribed, you should disable the heave and yaw DOFs at the seabed.
Best regards,