Nothing special will happen if you attempt a start with the gen switch switched into the on position.
The only thing that changes is that the generator will come online right after you release that starter button instead of when you manipulate the switch to the on position.
This isnt entirely without danger tho. This on its own isnt that dangerous, but it will cause an extra load on the engine and it might happen that the engine isnt powerfull enough at that speed, causing a drop in speed. This might drag the engine below the minimum self sustain speed, and if not taken care off, will give you a nice pudle of melted turbine parts.