The only dff to stl conversion or export we know about at this time, is potential export of RenderWare models (DFF format) for the purpose of 3D printing, that is to STL format.
We know that Blender supports .dff files and can be used for this purpose, but since export to STL format is supported in pretty much every decent 3D modeling tool, there are certainly more viable options how to convert dff to stl.