pre_comment_author_name.php

<?php
/**
 * Filter the comment author name for administrators
 *
 * @see wp_filter_comment(), sanitize_comment_cookies()
 * 
 * @param string $name The comment author name.
 * @return string The filtered author name.
 */
function filter_comment_author_name( $name ) {
  if ( current_user_can( 'manage_options' ) )
		return sprintf( __('Admin: %s', 'yourtextdomain'), $name );
	else
		return $name;
}
add_filter( 'pre_comment_author_name', 'filter_comment_author_name' );