What do you mean, specifically?
appdesigner calls a given callback function; that function can call anything, m-file or not; but the callback function itself has to be the specific one referenced to in the app.
You can't delete the callback function body automagically created when you define there to be a callback function and substitute a free-standing m-file function for it, if that is what you mean -- but you don't need to since you can immediately call whatever m-file that would be.
