Dear David,
You can use ServoDyn’s tower- and nacelle-based TMD submodules to add damping to the tower / support structure. The TMD submodules of ServoDyn contain all of the functionality of the old FAST-SC tool, plus additional features. While the page and documentation is a bit outdated relative to the new features introduced in FAST v8.15, the TMD webpage does provide some additional information: nwtc.nrel.gov/TMD.
Best regards,