Extending a chimney

Considering installing a wood furnace (Englander) in my basement. My current furnace is forced air LP so I could tap into the plenum so that seems pretty straight forward. I would like to extend the chimney of my wood fireplace (Intertherm) down into the basement to use it for the wood furnace. I could place the furnace directly under the fireplace so it would be a nice straight shot. The chimney also already has very draw. Any input, good or bad, would be appreciated.

Your Intertherm fireplace is a listed appliance, just like your furnace or washing machine. To modify or use it in a way it isn't designed for is usually not permissible by code. I've never considered doing what you suggested, but I can't think of a way to pull it off safely.

Agreed. If you want to install a wood furnace, have someone come give you a price to install its own Class A chimney.