If it’s a number issued by the International Maritime Organization for life, how would you do that? You can’t make up a number, and if you use a number used by another ship you’d have to make sure that your ship matches exactly the ship registered under that number so that you can fool anyone checking - and even then, that’d probably buy you a short amount of time but you’d still get caught.
How do you get a number assigned in the first place? I'd assume you'd have a shipwright just go through that process for a hypothetical ship they have "under construction" exactly matching your ship, then never build that ship, but instead assign its number to your ship.
