I've created and placed a new Building and Floor via GC Nodes and can see that data within the Floor Manager dialog. Can I modify properties for the Building and/or Floor while in the Floor Manager dialog?
No, if your Building and Floor were created using GC nodes then these nodes should be considered your "single source" of control from that point forward. This is similar to placing other OpenBuilding Designer components via Nodes such walls or doors, where it's only the Node that is used once placed. In the case of Buildings and Floors, if any properties are modified after the fact using Floor Manager, those modified property values will revert back to their GC Node values when the script is re-read; e.g., running the transaction, reopening GC, etc..
Note: If a Building or Floor is (for any reason) outright deleted from Floor Manager, attempting to perform further work within the same session may cause the node(s) to fail. If this occurs, simply exit from OpenBuildings Generative Components and restart - this will clear the local cached copy of the Building/Floor data and re-write the script contents to the FloorMaster.DGNLib file, thereby adding back the deleted entities.
GC SS6 - Creating and Modifying ABD Floors