How to add an inline word after post author commenter name?

Just wanted to mention some alternatives:

Alternative #1

You say you don’t want to edit the comments.php file in the current theme directory. If we need to change how the wp_list_comments() works, we can always modify it through the wp_list_comments_args filter. For example to add our own callback:

add_filter( 'wp_list_comments_args', function( $r )
{
    if( function_exists( 'wpse_comment_callback' ) )
        $r['callback'] = 'wpse_comment_callback';
    return $r;
} );

where the wpse_comment_callback() callback contains the customization of your comment’s layout.

Alternative #2

If we take a look at e.g. the Twenty Sixteen theme, we will see how the .bypostauthor class is used to mark the post comment author from the stylesheet:

.bypostauthor > article .fn:after {
        content: "\f304";
        left: 3px;
        position: relative;
        top: 5px;
}

where you could adjust the content attribute to your needs. This might not be the workaround for you, if you need to style it or use words with translations.