You mean you'll need an extra fan to help push the air out? I would rather pick a hood with a more powerful fan to begin with and eliminate the need for a second fan. An extra fan sitting in an inconvenient location in the exhaust path of all the cooking grease and soot is just asking for trouble down the road. Besides, modern hood fans are multi-speed which means they need a direct connection to the controller and you cannot just cut another motor into the circuit.
I can think of the ways to MacGyver an inline fan controller with a current-sensing transformer that would be completely external to the hood fan control circuit (current-sensing transformer positioned on its supply but not electrically connected) but I can't think of any way to do it without nullifying the warranty. Still, single powerful enough hood fan would be much better anyway.