Hello jrp-games,
I looked around, and I dont think this feature is present in Nabble.
I dont think it would be a good feature to add either, after all - whats to stop an admin from "reverse-spamming"?
What would make sense would be an option for users to tick a box stating "Willing to receive emails from administrators" when registering. And that giving you (the administrator) access to a list that you can email.
At present, the Nabble team could make this change for you if you post to the
premium support forum.