That was done manually, because that "black" subfolder is a black skin of the BMW model. So I just needed to create the subfolder and put files that define what's been changed there in relation to the base model. In this case it was just the new matlib file (and the new objdef file itself), but you can also change other files there, textures, scripts, all except the geometry pkg file. What's not found in the skin folder is taken from the base.