Can you convert an openside fireplace to a closed one?

Our lower level has a fireplace that is open on the right side and i would like to surprise my husband by having it closed it possible. With winter coming and last year we lost power for 3 days it would be nice to have fireplace that actually provides heat. We had to get the fire really big to get a little heat. When it was cleaned this spring the chimney guy said that we need to close up the side. ANY suggestions??