Custom ground skins work the same way as any other skin, just import the .blp, and change the directory to what you want to replace. Then close world editor and open it back up again and load your map. That should do it as long as the .blp isn't damaged and the directory is correct.
I just checked warcraft 3 viewer, and all the terrain skins it has are all .blp. Also i saw a map with a custom tile, and when i exported the tile, it was .blp.