I have a cottage that has a gas heater on the first floor and then just a register in the ceiling of the first floor (which is the second floors floor). The heat passively migrates to the second floor.
They make ones that are decorative for this purpose. Its a simple and inexpensive solution. If you are talking about forced air then you would need to consult your pellet stove manufacturer to see if they have blower kits that would support ducting.