There needs a way to add subscribers to a newsletter manually. Now the only way is to expect users to subscribe themselves, which is good as a default use case but limits two other valid use cases:
* Migration of existing lists of users in other formats (MassMessage, wiki lists...)
* Possibility to add manually users by admins, under admin's criteria (probably more relevant in 3rd party wikis than Wikimedia) i.e. a corporate wiki.
This feature should be available for administrators only. When abused, this is basically a powerful tool for easy spamming. Considering how easy is to become the owner of a newsletter and how infrequent is the need of this feature, I think restriction to administrators is the way to go.
=== Old description ===
A way to easily help readers migrate to the new system, without them having to do anything.
* Develop a way for admins (or confirmed newsletter publishers) to migrate the list of existing subscribers (either raw wikitext lists, or a massmessage listing) into the new system - (??)
* ...
* Profit!
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For the initial newsletters that use this system, and/or old newsletters with massive subscription lists, this will also need: First get community consensus (or per-newsletter consensus) that this extension is a good replacement for all stakeholders