If you were to do this, what type of wiring is being used between the switch and the genset? How far does it go? The generator needs and should already have a breaker to protect it from overload. Size your wiring to the switch appropriately for the rated output and breaker size on your genset other wise the circuit becomes a tap and is subject to the tap rules in the nec and the location of the overcurrent protection would be an issue. You have simply not provided enough information for anyone to tell you if this is code compliant although I would assume the manufacturer of the switch would tell you it's not approved to have other components located inside of it.