Use war3 model editor, open it, go to texture manager, right-click the texture, export, change the extension to .blp
I think he wants to make one of his textures a .blp and not a blizz one.
Or you could get Photoshop. solves all your problems.
My photoshop couldn't convert to blp.
Use war3 model editor, open it, go to texture manager, right-click the texture, export, change the extension to .blp