Dear Roberto,
Yes, that is correct, except that OpenFAST also supports a delta-3 angle, which means that the teeter axis must be perpendicular to the shaft, but need not be perpendicular to the blade axes.
To include additional teetering DOFs, e.g., a ball joint instead of a pin joint, would require updating the kinematics and kinetics expressions within the ElastoDyn module; this would also impact the formulation of the kinematics and kinetics of the blade degrees of freedom (DOFs). I’m sure this is possible to add, but will not be simple. I would suggest to start by reviewing how the existing teeter DOF is implemented in ElastoDyn. This is documented in what is called the “Unofficial FAST Theory Manual” on this forum (which is effectively the unofficial ElastoDyn Theory Manual): FAST theory manual. That said, if the frequencies of these new DOFs are high relative to the rotor speed (due to the high stiffness you mention), I would question their impact on the loads calculation.
Best regards,