Solder two leads to the existing switch. You are looking for two small thru holes pretty close to the switch, they use them for testing. Solder the wires to the holes, run the wires to a small hole, or connector placed in the casing of the laptop (something you put in yourself.)
Connect a momentary switch to another set of extension wires running to the connector. Enjoy!
P.S. B very careful soldering, or you might not like this method....