Dear Jharna,
If all your interested is the mass, center of mass, and inertia, then only the mass matrix resulting from the FAST v7 linearization calculation is important. The settings of wind, waves, control inputs, wind-input disturbances, etc. shouldn’t matter. Simply disable all DOFs except the platform DOFs (PtfmSgDOF = PtfmSwDOF = PtfmHvDOF = PtfmRDOF = PtfmPDOF = PtfmYDOF = True), set all of the masses you don’t want to zero (e.g. PtfmMass = PtfmRIner = PtfmPIner = PtfmYIner = 0), and set the masses you do care about nonzero. Linearize about the initial conditions (CalcStdy = False) and look only at the resulting 6x6 mass matrix, which will contain the mass, center of mass, and inertias of the remaining mass about the platform reference point.
Best regards,