Is there an extension that allows me to add comments inside indiviual customers in the "Manage Customers" section?
You can try our Customer Attributes solution allowing to add any additional information to personal customers' accounts, including comments, photos, and anything you need. All the information is displayed in the Manage Customers section.
Handy for both customers and admin to fill and read the information.
You just need to create an attribute for Comments for the customers and you add comment in this attribute.