If you want to add them before, you can try to use the World Template feature. With that feature you should be able to create a world and use it as a template for creating worlds.
I don't know other ways to do it, because it was never intended to use custom blocks.
this plugin would be very popular if it's used like the server OriginRealms, i'm afraid to make it permanent on my server and bugs not get fixed lol this is the time i wish how to code