not sure what you mean when you say surface assignment but if you mean the texture does not go onto the newly created faces ..then you have to re UV map the surface to include the new faces.
If you are talking about your imported car from Lightwave then you need to learn about UV mapping.
In order to texture a Polygon object in Maya ,first you have to do a projection map of the UV's and lay them out flat in the 0-1 texture space of the UV Texture Editor.