Dear Lingling,
You can find formulas for computing inertias of rigid bodies in mechanical engineering textbooks. See my Nov 23, 2009 post in the forum topic found here for an example where I convert the inertias from the tower-top / yaw bearing to the center of mass of the rotor-nacelle assembly: Tower fore-aft modes shapes.
In BModes, if beam_type = 2, hub_conn = 2 or 3, and tow_support = 1, than yes, the 6x6 stiffness matrix at the platform reference point, mooring_K, will be used. My understanding is that BModes simply sums mooring_K with hydro_K.
Best regards,