Dear Riccardo,
The linearization for floating offshore wind systems was not available in FAST v8, but was recently introduced in OpenFAST–see: github.com/OpenFAST/openfast/pull/350.
The FAST v8 and OpenFAST linearizations do not show the mass matrix directly, but this matrix can often be inferred from other matrices generated through the linearization process. A similar question was asked and answered in the following forum topic: OpenFast 2nd order Linearization - #2 by Jason.Jonkman.
Best regards,