Dear Guillermo,
The introduction of coning would add a COS(PreCone) factor multiplying Omega^2 in Eq. 3.77 from that report.
That said, Eq. 3.77 is not actually implemented in FAST, in contrast to what is implied in that report. Instead, FAST implements all inertia-related terms through direct implementation of the kinemetics, which are based on the specified turbine geometry and all system degrees of freedom. But these equations are far beyond what can be written in a forum post. They are documented only in the “Unofficial FAST Theory Manual,” mentioned in this forum post: Coupled blade modes in FAST.
I hope that helps.
Best regards,