In order to add an addendum to the closed document, you will need to:
Scroll to the bottom of the form and click +Add Addendum
A modal will pop up, this is where you will select the type of addendum from the drop-down and enter your content.
Your success message will appear and that's it! You're done.
Note: addendum's are only available to be edited within the first hour of adding them. After that they are permanent.