Dear Gia,
The tower-top mass/inertia properties of the NREL 5-MW turbine are documented in the following forum topic: Using Aggregate Mass in ADAMS to Check NREL CS_Monopile.bmi.
Your understanding of how to change the tower thickness of a FAST model is correct–that is, re-derive the distributed tower mass and bending stiffness and recomputed the tower-bending modes (via BModes).
Best regards,