Ignition switch has no accessory position. Can i replace it with a switch with an accessory position?

"On" is not the same as accessory position! According to the owner's manual for the 2014 F-150 the ignition switch, like most, has four positions: "Off" - "Accessory" - "On" - "Start". Off is the one where you can remove the key. Accessory turns on power to the radio and other systems. On powers the ignition and the gauges come to life. Start is spring-loaded and cranks the engine.