If you textured your 3DS correctly, you don't need to use entitytexture as the texture is already present on the object.
If it isn't then it is either exported incorrectly or the texture is not in the same directory as the mesh. (try loadanimmesh to make sure it isn't some strange behavior from a different thing)
If it isn't on it and you want to manually texture it, you must use the paintbrush approach to texture each surface manually (entitytexture does not do that, it textures the whole mesh).
So it sounds more like you are not understanding your problem correctly at the moment if you think you need to use entity texture to texture a pretextured object :-)
On the AddVertex thing: You can't change the surface of a vertex. And it is not needed anyway. Any change on a surface means full reupload of the whole surface to graphic card anyway.
If you want to see whats within a surface, you can use CountTriangles and TriangleVertex and the VerteX / Y / Z commands
If it isn't then it is either exported incorrectly or the texture is not in the same directory as the mesh. (try loadanimmesh to make sure it isn't some strange behavior from a different thing)
If it isn't on it and you want to manually texture it, you must use the paintbrush approach to texture each surface manually (entitytexture does not do that, it textures the whole mesh).
So it sounds more like you are not understanding your problem correctly at the moment if you think you need to use entity texture to texture a pretextured object :-)
On the AddVertex thing: You can't change the surface of a vertex. And it is not needed anyway. Any change on a surface means full reupload of the whole surface to graphic card anyway.
If you want to see whats within a surface, you can use CountTriangles and TriangleVertex and the VerteX / Y / Z commands