See nuevoMailer WP, a new product which is a fully featured WordPress plugin.
Plugin for nuevoMailer SB
you can create a subscriber in nuevoMailer with the name and email (and more) of anyone who posts a comment in your WordPress blog. You can create the subscriber either when the comment is made or when you approve it. In addition, you may use double opt-in, send a welcome email etc. This plugin is part of nuevoMailer since v.7.5.
Embed opt-in forms in your WordPress pages
You can embed a nuevoMailer opt-in form in a WordPress page like you do with any other page.
Copy-paste the form html code or
Use an iframe to embed it.
Here is video demonstrating the iframe method.
Both methods work fine. With (1) the form will inherit styling from WordPress. With (2) you have also the option to add a captcha in your form.
Import your WP users emails, names and more using an external database connection. In your custom sql you can filter on user_status in case you want to differentiate them among lists and/or focus to a specific user group. See how to map your table and fields.
Remember that you can have your external database connections automatically and periodically synchronized using a cron job so that your nuevoMailer database is always synced with WordPress.