The Import Users to MailChimp allows you to quickly and easily import all existing users to mailchimp.
Entering your MailChimp API
Selecting your MailChimp lists to Automatically add, remove, and update users to your MailChimp mailing list as users subscribe and unsubscribe to your WordPress site
Import all existing usesrs to selected MailChimp lists
Configuring extra fields on your Signup Form and also labels