You would have to change the blight texture by replacing it with another via importing it. If the wrap is the same, you could just extract the snow texture and upload it to you map with the blight texture's file path, and it will automatically overwrite the blight with snow in your map. If the wrap is not the same, you'd have to extract the blp's, conver them to jpg/tga, cup 'n past the snow over the blight and resave as .blp and upload into your map.
-VGsatomi