0xRH's GBA Pokémon Game Editor has an inbuilt Pokedex order editor, it's the bottom list I think (the top one is National Dex Number, might be other way round). I'm sure there are standalone tools but that's what I've used.
*EDIT* I recommend looking in resources and tutorials for how to extend the regional dex if you plan on adding more to it. I'm too lazy to link but it should be easy to find