I found the attached drawings in
Producing You Own Power (edited by Carol Hupping Stoner), in the chapter on cooking and heating with wood. It's a wood stove made out of two 55 gallon drums, the bottom one for burning, the top one is a heat chamber. The latter can have a smaller (35 gallon) barrel inside, fitted with a door, and used as an oven, or just for extracting more heat from the flue (more surface area).
So I'm thinking you could add something like that to your stove's flue. I imagine you could put bricks or water in the upper barrel to keep heat stored for longer...