Dear Aitor,
As discussed in the following forum topic: How to find the paramter of the first bending modal mass, damping, and stiffnees coefficients? - #2 by Jason.Jonkman, FAST v8 returns the linearized model in first-order form. So, while you can’t get the second-order mass and stiffness matrices (M and K), you can get the same terms normalized by the mass matrix i.e. [ M^-1 ]*[ M ] = I and [ M^-1 ]*K through the FAST v8 linearization analysis. This first-order form should also be amendable for controls design.
That said, a linearization analysis with FAST v7 can be used to obtain the mass and stiffness matrices.
Best regards,