What you are looking for is a dnn user profile import. The cart doesnt come with a button to import customers to the dnn user profile if you search snowcovered using the keyword "user profile import" you will find many modules that allow you to import an xls or csv to the dnn user profile. Once your customers are imported to the dnn user profile the cart will automatically retrieve their information and auto populate the checkout screens if they are logged in.
-Scott