add_filter( 'the_content', 'sd_teaser_in_archiven', 5 );
function sd_teaser_in_archiven( $content ) {
    if ( is_admin() || is_singular() ) {
        return $content;
    }
    $excerpt = get_the_excerpt();
    if ( ! $excerpt ) {
        return $content;
    }
    return wpautop( $excerpt )
         . 'Weiterlesen →';
}