Add an email notification?

Hi,

I'm not sure what you mean.
When you change the status of an order in the backend order management, you have a checkbox to notify the user. If you activate it, the "order status notification" will be sent to the customer, regardless of the status of the order.
Similarly, if you use an action "change status" in a mass action, you have a checkbox to notify the customer. It will also use that "order status notification" email for that.
Also, via the System>Emails menu, you can personalize that "order status notification" email and you'll find "per status override" buttons there to be able to customize the email differently for each order status.