@bob what you can do is what I do. Just create a temporary mailing list and add those new subscribers to the list. Then copy (Duplicate) your old Newsletter and resend it to those subscribers.
Yeah, it's a pain. And you'll probably have to keep doing this many times. But currently it's the only way.
Unfortunately, this feature has been requested here over and over. They keep saying they are thinking about it and that they have forwarded the requests to the developers. Hopefully they added before we lose all hope 🙂