Is there a mod already built in

Web Wiz Forums by default will add a comment to say which member edited the post and when.

If you want to add some text which says 'Edit by Admin' you would need to add the text manually at this time.

Not sure if it is as simple as that, whenever a post is edited a small piece of XML type code is added into the post, this is then parsed in different ways depending how and were the post is displayed to say who and when the post was edited.

No a 3rd party program will not change this as you would need to modify the code within the forum software so that the data you need is added to or attached to each post.

The best way would more likely be to add a new table to the database to store comments then link it to the tblThread table.

You would also need to write all the frontend to be able to add and display this code. It's allot of work so hope you have weekend free to implement it.