Depends on if the object actually uses multiple textures. If it doesn't, the setTexture-method in Object3D is sufficient. If it does, you have to use the PolygonManager to achieve this. Just iterate over each polygon, get the texture id and the u/v coordinated for each vertex, create a new TextureInfo with that, add you additional texture to it and re-assign it using the PolygonManager. It's a bit awkward though.