However, in the future, why not skip the step of manually exporting and appending those contacts to your existing list... Wouldn't it be nice if the list would just grow on it's own, and automatically filter out duplicate entries?
Creating a Sign Up form in Mail Chimp and adding it to your website as either a landing page or as a link saves you that extra step. Don't forget that you can also check out the FREE automation feature in Mail Chimp so you can even send a personalized welcome message to your new subscribers.
Speaking of which you can try out mine if you like: Officeology Sign Up
https://community.weebly.com/t5/Promote/Export-a-Contact-Form/td-p/17682