You can do it by using the 'Change Party Members' event that can be made in a common event and then called or simply in any normal event. Use conditional branches for limitations. Sorry, Economics mid-terms coming up within 15 hours; no time to provide details atm.
There's also a pokemon-styled easy-party-switch or something like that somewhere in the script database; if it isn't here then it probably is on hbgames.org.