Mixing the flue gasses is a terrible idea. You'll create a chemical reaction in the chimney that can cause damage to the structure or yourself.
It sounds like you are asking about an emergency situation in which case you probably wouldn't have the water heater running. In that scenario I would have some extra vent tube and elbows available to construct something to go in the existing (or enlarged) hole. I wouldn't recommend installing anything before you need it, because realistically you'll probably never use it. It could also end up flooding your basement with carbon monoxide.
It sounds like you are asking about an emergency situation in which case you probably wouldn't have the water heater running. In that scenario I would have some extra vent tube and elbows available to construct something to go in the existing (or enlarged) hole. I wouldn't recommend installing anything before you need it, because realistically you'll probably never use it. It could also end up flooding your basement with carbon monoxide.
