You can buy switch units that plug into a servo channel but the easiest method is to use a small servo to push/pull a switch on. You can have a switch that pulls/pushes the switch or make a servo wheel into a cam that pushes on a micro switch arm.

For simplicity, a small servo acting on a proper switch is the quickest solution.....unless you happen to find one of these electronic switches that suit RX's (not a normally stocked item and often limited by electrical load). I'm not knocking the electronic switches but you need to ensure you aren't overloading it.