Adding another command in Thread Tools
 

In the showthread.php, there is a "Thread Tools" button where you get an option to "close thread, delete thread, merge thread, etc."

I want to add another one called "Post Expired"

once that is done, I would like it to show an expired icon on the post, just like how when a thread is closed, there is an "lock" icon. Which files or templates do I need to modify? Thanks for any pointers.