Mikko Virenius

This WordPress Plugin allows an admin user to add comments even if the comment thread is closed.
Features
<ul>
<li>No options panel</li>
<li>No extra configuration required</li>
</ul>

Admin Can Always Comment

I use this on CSS-Tricks for the obvious reasons. I close comments after a certain amount of time, but as an admin, I want to be able to leave comments whenever I want to. 

Works Great with 4.9.6

This plugin allows an admin user to add comments even if the comment thread is closed.

Rapidweb

Edit user permissions for Ninja Forms.
By default, all Ninja Forms options are only available to users with the ‘Administrator’ role. This plugin will enable you to set which elements of Ninja Forms different user roles should have access to.

Admin Can Always Comment vs Permissions Editor for Ninja Forms