dff to dae conversion is typically searched by users looking for a way to export RenderWare models (.dff) to Callada 3D format (.dae), typically for modding or exchange purposes.
Blender seems like the popular choice among modders and you can download and use it for free, so it's probably worth a try. You can find numerous guides and tutorials for in on the Internet and these might help you assist with your export goal.