Anyway thanks for the answer :)
You don't actually have to put it in the script as it can be activated at any time. In fact, it wouldn't be wise to paste it into Oak's script as you'll have to repoint the whole thing otherwise you'll be overwriting something (this is because you would be making the script longer by inserting an extra command). I would advise you to make one or two green script tiles that the player has to walk over that uses the special command before they are able to capture non-Kanto Pokemon. This way you won't have to alter Oak's script and you'll still be able to evolve your Pokemon.