To add an option in there, you need to create a code event listener using the template_post_render OR template_hook if the template have a hook.
If you want to add an option in the forums edit page, you need to look for a hook inside the template forum_edit. This is an admin template, so you will look for it in the "Development" tab and then "Admin Templates".
After you take a look and realize that there is a hook, you create a code event listener to the template_hook and add your option.
Take a look at the DataWriter, that's where the magic happens.
The _setFields()-method is where you define the new column and you can then use the DataWriter (inside the controller) to save the information to the database.
Sorry, I don't have access to the code right now but that should give you a hint.