Dear Leanardo,
When I originally developed the YawInertia for the NREL 5-MW baseline wind turbine, I used the linearization functionality of pre-FAST v7 (which is similar to that of FAST v7). The yaw inertia comes directly from the mass matrix (no MBC3 needed) for a model with only the nacelle-yaw DOF enabled.
The FAST v8 and OpenFAST linearizations do not output the mass matrix directly, but this matrix can often be inferred from other matrices generated through the linearization process, e.g., as discussed in the following forum topic: OpenFast 2nd order Linearization - #2 by Jason.Jonkman. A process similar to what is described there could be done to derive YawInertia when only the nacelle-yaw DOF is enabled.
Best regards,