It appears that the ZModeler program allows users to import RenderWare models (.dff), work with them or save them in ZModeler's default .zd3 files, which should be perhaps enough for dff to z3d conversion. This might actually be the only way how to perform this 3D graphics conversion.
Open DFF file Open Z3D fileA 3D Editor used to create custom add-on objects to Direct3D based games