Ok I see what you mean. I thought you were referring to the terrain of the scene, but you mean the bottom portion of the panorama where a grid would normally be in the software. correct?
I think you can create your own panorama pictures to use. Just click the panorama button ..
..and then Browse your workspace for whatever picture you want to use for an Environment Map . The bottom portion of the picture would be what appears to be the texture of the "Grid" area. (see pic below)
So you could change the bottom half in photoshop or powerpoint and then use that NEW image as your Env. Map.