Here's a quick way to import your new customers to your Mailchimp newsletter list.
Go to View Customers, then click on the reports dropdown on the right and choose New Customers.
Click on the Date Created column header to sort the list by the created date. Then simply copy the customers who are new since you last added users to Mailchimp.
In Mailchimp, choose the list to add the new customers to. Click Add Subscribers, then Copy/Paste from Excel.
If you would like more control over exactly what gets imported, or need to format things BEFORE you import, you can also download the customer CSV list from the View All Customers page.
There is a 'Date Added' field you can use to sort the list by the newest customers, and then edit the list as you need in Excel.