One thing is for sure, that style BBQ cooks great and you can easily smoke food by adding wood chunks to the charcole. The Easiest thing to do would be to set the BBQ up for indirect grilling (keep the flames away from the food) Once you get the temp your looking for then add your soaked wood chips to the charcol.....thorw the lid back on......and you have your smoker.
Other then that Im not 100% sure on how to convert that into a "barrell" style smoker.