Dear Jason,
Since the release of FAST v8.16, we’ve added the option for a new user-platform module (ExtPtfm) that will allow you to implement user-specified 6x6 mass, damping, and stiffness matrices, and a 6x1 load vector to be applied to the platform in ElastoDyn (or a more complicated user-specified platform load). This feature will allow you to import a superelement (expressed in terms of mass, damping, stiffness and load) for use within FAST. The new release of FAST–with this and other features–is imminent.
Regarding soil-structure interaction, this has been discussed a few times on this forum e.g.: Foundation Stiffness and damping in FAST v8.
Best regards,