You can try loading the .dfs file in BrainSuite and then save as .obj file. It should save outputs in MNI obj format.
If you need to get the file into the Wavefront obj format, there is a trick you can try. Save the file using a .wfo extension, and BrainSuite will write it out in the Wavefront OBJ format. You will then need to change the extension manually to be .obj, but the file should be readable by programs expecting a Wavefront OBJ file.