It is not practical to set up a separate mail list on Nabble as it will create constant administrative burden ensuring that the list of users on one application echoes that of another, and your members would still have to accept membership of that maillist.
I'd recommend this course of action:
1. You can't make certain messages in a forum mailable. It's all or nothing. That means you need to create a sub-forum where you post your message to be sent by mail.
2. You can't force forum users to take emails. The best you can do is invite them to subscribe to your sub-forum. Further, to avoid anyone spamming others, Nabble demands that Administrators the request the "Invite Subscribers" module. (See: OPTIONS > USERS> MANAGE SUBSCRIBERS) You need to post a message here quoting the URL/permalink of the forum that requiring the facility, then you will be able to issue the invitations, which users need to respond to by clicking a link. (Of course, they could always opt to receive your mail themselves by selecting OPTIONS > SUBSCRIBE VIA EMAIL)
A further possibility would be to start using the "Member" group. You need to add your users to this manually, and you would only do so if your "Registered" users opt to subscribe to the sub-forum. Currently, there's no way to be notified of that, so that would still require constant monitoring of the subscriber list.
If none of these options is satisfactory, then it is of course, possible to edit the NAML code behind your forum. I don't have the skills to advise on that.
Finally, I know that for a Premium Support subscriber Nabble were able to add an "Add Subscribers" module (that doesn't require a user's acceptance) to that user's forum, so it may be possible to have that installed on upgrade.
Note to Nabble: It might be an idea to add a "Notify me when someone subscribes" facility to the OPTIONS > APPLICATION > EXTRAS & ADD-ONS > EMAIL AND NOTIFICATIONS panel.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.