It is possible to create custom meal types yourself, and you can rearrange the order as much as you like (though there is, of course, a natural order in the premade meal types).
However, only the six premade meal types will work with the Food Diary.
Bonus! If you, as a Coach, have your own Custom app with us, you can try reach out to us and see if we have the option to rename one of the premade meal types. This means the name will appear as desired for your clients, but it will still work with the Food Diary - this is often one of the 'snack' meal types that get renamed.