you import it to WE (yeah...)then rename it like TerrainArt\Icecrown\Ice_DarkIce.blp (can use any, one is usually given when u dl it) and then go Advanced and go Modify Tilest , tick the box and replace any tilest with the one u renamed such as TerrainArt\Icecrown\Ice_DarkIce.blp would be in ICECROWN AND WOULD BE DARK ICE.
hope that helps u