Dear @Achilleas.Kazerakis,
See a similar question asked and answered in the following forum topic: Is there any subroutine to mannually assign distributed loads in Subdyn. In short: while SubDyn currently supports apparent fixity (AF) and coupled springs (CS) representations of the foundation, OpenFAST does not currently support a distributed springs (DS) representation of the foundation. That said, there is placeholder for future development of a DS model in the upcoming release of the SoilDyn module (but source code changes would be needed to implement that).
Best regards,