How can I sync the plugin with the database I already have since the plugin only fires when an action happens like new registration?

We have not implemented full initial sync into this plugin because it’s extremely easy to do this with existing tools. You can simply export your users to CSV (there are a number of free plugins to do this) and import it into MailChimp.

P.S: Just don’t forget the ROLE field.

Once you have the initial sync done, Chimpy will make sure all the new users are added automatically. You can read more about importing data into MailChimp here: http://kb.mailchimp.com/article/how-do-i-create-and-import-my-list/#import