...... They (pellet smokers) are designed as a pressurized system to attain the right airflow/fuel mixture, if you add an extension it will cause a draft and mess up the whole burning process. Its also not advised to add gaskets to the lid either, this is a potential fire hazard to auger pot supplied burners. The "losses" through he lid are part of the design and help reduce a blow back fire into the auger tube and hopper.Why don't you recommend an extension?
Like others have said, it would be better to add an exhaust fan or an outdoor hood/fan about 30-36 inches above the smoker.