Hi @Oscar ,
.mobj is IPGs proprietary encrypted 3D model format. Conversion from obj to .mobj is unfortunately not possible from your end.
But the good news is, you can integrate your custom 3D models through the glTF format. A detailed description of the steps on integrating can be found in the MovieNX manual (CarMaker GUI → Help → MovieNX Chapter: Integrating Custom 3D Objects).