Snap-Lock Black Steel Stovepipe 8-Inch Male to 6-Inch Female Reducer

It is never a good idea to vent into a larger diameter chimney for an appliance that is designed to use a 6" chimney. This will slow the velocity of flue gases as they exit the chimney system to establish draft, resulting in the buildup of creosote and performance issues with the appliance. This is especially the case with modern high efficiency stoves that are particularly sensitive to draft issues. In addition, you would need to use an Increaser to maintain the "male end down" connection to the stove.