I'd rather not edit theme. Please suggest some hook or snippet to get it done individually rather than modifying existig code.
You can utilize the_publish action hook or filters such as the_content or even thename, it is dependent on which for you to do.
The entire listing of all hooks: http://adambrown.info/p/wp_hooks/hook
About while usingpublish hook: http://wordpress.org/support/topic/how-to-use-the_post-action-hook