Adding a filter to my posts

The excerpt must be used by the Theme. I don’t think it is. Use ‘the_content’ filter to append to the post. Excerpts are usually used in feeds and only by certain themes.

add_filter('the_content', function($content){
    if(!is_archive()) return $content;
    global $post; // This is the current output post
    ob_start();
    // Echo stuff to append to post here! (your link or whatever)
    return $content.ob_get_clean();
});

PHP 5.3 Closure used. Extract function yourself aside for PHP 5.2.

Regards.