you never mentioned about 'opening' it, you asked.
wii backup manager can do that for you.
make sure the .wbfs and .wbf1 are named the same and in the same folder, launch WBM, under 'inactive' choose your drive / custom folder and it will mount it and show any / all games present.
click the checkbox and highlight the (large) game you require, then Transfer > ISO> choose where you want the ISO and WBM will make it there