how can I add products to an order in the back-end

Sometimes I want to change an order a customer made. I would like to add existing products. But now I just can add a product I fill out the fields for.
like this:
screencast.com/t/1zQlA1ThoN5U

Is there a way for me to add existing products to an existing order?

Hello,

In the order details, for the product listing, you have two icons.
One is to add a "custom" product (the plus icon) and the other one to add an existing product (the package icon).
Via that second button, you would have to select the product and then you will have the interface of your screenshot (but with the fields pre-set).

Regards,