Converting an OBJ to DFF is never a one-click operation. It is a translation from a neutral, modern format to an aging, game-specific binary format that enforces strict rules. The process forces you to understand how RenderWare thinks: hierarchies, material splits, collision lumps, and atomic flags.
Design for Fixturability (DFF) Methodology for Commodity Parts convert obj to dff
There is no single official converter. Below are the three most reliable pipelines used by the GTA modding community. Converting an OBJ to DFF is never a one-click operation