Adding more text to a post, after it was published

Your best bet is to use the the_content filter hook and add your content to the post
“on the fly” for ex:

add_filter('the_content','add_my_extra_content');

function add_my_extra_content($content){
   $my_extra = "<h5>this is the extra content</h5>';

   //add before post content
   // return $my_extra.$content;

   //add after post content
   return $content.$my_extra.;

}