If running FPP as master, just use the "Big Buttons" plugin to launch commands to insert the desired songs. It's very simple.
That said, FPP also has a very extensive REST api so you could easily have something that just sends off REST calls to start the various sequences or whatever depending on the button pressed. If you go to http://fppipaddress/api/help.php you can get an idea of the various API's that are currently available, with more being added all the time. Almost all of FPP's REST api's and Commands are also available via MQTT so if you have something that can fire off mqtt events, that could be usable as well.