Dear Alessandro,
Ah, OK. In this case, FAST is essentially treating the total tower top (rotor, drivetrain, nacelle) as a rigid body in the linearization analysis. I would encourage you to use option (1) because you are already familiar with the FAST v7 linearization process and to avoid the complexity of using MSC.DAMS. That is, set the tower and platform masses and inertias to zero in the FAST input files and linearize the FAST v7 model with only the six platform DOFs enabled using MdlOrder = 2. The resulting 6x6 mass matrix will then represent the rigid-body of the total tower-top (rotor, drivetrain, nacelle) about the platform reference point, including the total mass (upper-left quadrant), center of mass (upper-right and lower-left quadrant) and inertias (lower-right quadrant).
I hope that helps.
Best regards,