Dear Yingyi Liu,
Yes, your understanding is correct.
The total integrated force and moment from the ballasted members are written to the HydroDyn summary file, but the center of buoyancy is not. From this information, you can manually calculate the transverse offset of the center of buoyancy of the ballasted members (xf and yf), but not the vertical center of buoyancy (zf), as discussed recently in the following forum topic: Hydrodynamic analysis of OC3-Hywind spar buoy - #42 by Jason.Jonkman. However, for simple cylindrical geometries, it may not be difficult to calculate the center of buoyancy manually yourself. In addition to defining the ballasted members in HydroDyn, the only additionally thing you need to do is to modify AddCLin.
We do not currently have funding to extend HydroDyn to output the center of buoyancy etc. from HydroDyn. Of course, the source code is open, so, you or others could make these changes yourself, if needed.
Best regards,