over writing pingback.ping with xmlrpc_methods

is it possible to overwrite functions using xmlrpc_methods? I want to add an extra field to pingback comments

function attach_new_xmlrpc($methods) {
    $methods['pingback.ping'] = 'new_pingback_ping';
    return $methods;
}
add_action('xmlrpc_methods', 'attach_new_xmlrpc');

function new_pingback_ping(){
identical to ABSPATH . xmlrpc.php, but with an extra line.
}

But when I do trackback seem to stop working completely.