I have the same bike and have added a kill switch. There are 4 wires that come out of the key switch, can't remember the colors but you can wire two of them to a kill button and leave the other two on the key switch if you want the key switch to control when the starter button is live. For the kill button you want the two wires that have continuity when the switch is off. Otherwise you can wire the other two together and the start button will remain hot at all times and you can removebtevkey switch entirely, or you can put some other switch between them.
The wiring diagram for the 2011 six days model, I think it was, helped me a lot. You can find it on the gas gas Spain website. Take the key switch off the bike and test the wires with a continuity tester and the key switch in off and on position. You want the two wires, for the kill switch, that have continuity when the key switch is in off position.