It seems the ZModeler program can import RenderWare models (*.dff), work with them or save them in ZModeler's default *.zd3 files. Probably one way how to perform this 3D graphics conversion.
Convert RenderWare model file to ZModeler 3D graphics format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert dff to z3d on Windows
A 3D Editor used to create custom add-on objects to Direct3D based games
Convert from dff file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.