Although this has been requested many times, there are only workarounds to what you want, I think.
- Manually export the user list and import it to your mailing provider
- Sync your users to Wordpress via SSO, and then sync them from Wordpress to Mailchimp (or whatever mailing provider you’re using.
- Use this (currently broken) plugin that adds a webhook action on new user signups, and make Zapier add the user to whatever provider you use.