¿Cómo añadir un campo NIF o CIF con validación a WooCommerce 2.4?

//Arreglamos la dirección predeterminada
function campos_de_direccion( $campos ) {
    $campos['nif'] = array( 
        'label' => __( '<abbr title="Código de Identificación Fiscal" lang="es">CIF</abbr>/<abbr title="Número de Identificación Fiscal" lang="es">NIF</abbr>', 'woocommerce' ),
        'placeholder' => _x( 'Introduzca el CIF/NIF', 'placeholder', 'woocommerce' ),
        'required' => false,
        'class' =>array( 'form-row-last' ),
        'clear' => true,
	);
    $campos['email'] =array( 
        'label' => __( 'Email Address', 'woocommerce' ),
        'required' => true,
        'class' =>array( 'form-row-last' ),
        'validate' =>array( 'email' ),
        'clear' => true,
	);
    $campos['phone'] =array( 
        'label' => __( 'Phone', 'woocommerce' ),
        'required' => true,
        'class' =>array( 'form-row-first' ),
	);

    $campos['company']['class'][0] = 'form-row-first';
    $campos['city']['class'][0] = 'form-row-first';
    $campos['state']['class'][0] = 'form-row-last update_totals_on_change';
    $campos['postcode']['class'][0] .= ' update_totals_on_change';

    //Reordenamos los campos
    $campos_nuevos['country'] = $campos['country'];
    $campos_nuevos['first_name'] = $campos['first_name'];
    $campos_nuevos['last_name'] = $campos['last_name'];
    $campos_nuevos['company'] = $campos['company'];
    $campos_nuevos['nif'] = $campos['nif'];
    $campos_nuevos['address_1'] = $campos['address_1'];
    $campos_nuevos['address_2'] = $campos['address_2'];
    $campos_nuevos['postcode'] = $campos['postcode'];
    $campos_nuevos['city'] = $campos['city'];
    $campos_nuevos['state'] = $campos['state'];
    if ( isset( $campos['email'] ) ) {
		$campos_nuevos['email'] = $campos['email'];
	}
    if ( isset( $campos['phone'] ) ) {
		$campos_nuevos['phone']['required'] = true;
		$campos_nuevos['phone'] = $campos['phone'];
    }
	
    return $campos_nuevos;
}
add_filter( 'woocommerce_default_address_fields' , 'campos_de_direccion' );

//Nueva función para hacer compatible el código con WooCommerce 2.1
function dame_campo_personalizado( $campo, $pedido ) {
    $valor = get_post_meta( $pedido, $campo, false );
	if ( isset( $valor[0] ) ){
		return $valor[0];
	}
	return NULL;
}

//Añadimos el NIF y el teléfono a la dirección de facturación y envío
function anade_campo_nif_direccion_facturacion( $campos, $pedido ) {
    $campos['nif'] = dame_campo_personalizado( '_billing_nif', $pedido->id );
    $campos['phone'] = dame_campo_personalizado( '_billing_phone', $pedido->id );
	
	return $campos;
}
add_filter( 'woocommerce_order_formatted_billing_address','anade_campo_nif_direccion_facturacion', 1, 2 );

function anade_campo_nif_direccion_envio( $campos, $pedido ) {
    $campos['nif'] = dame_campo_personalizado( '_shipping_nif', $pedido->id );
    $campos['phone'] = dame_campo_personalizado( '_shipping_phone', $pedido->id );
	
	return $campos;
}
add_filter( 'woocommerce_order_formatted_shipping_address','anade_campo_nif_direccion_envio', 1, 2 );

function formato_direccion_de_facturacion( $campos, $argumentos ) {
    $campos['{nif}'] = $argumentos['nif'];
    $campos['{nif_upper}'] =strtoupper( $argumentos['nif'] );
    $campos['{phone}'] = $argumentos['phone'];
    $campos['{phone_upper}'] =strtoupper( $argumentos['phone'] );
	
	return $campos;
}
add_filter( 'woocommerce_formatted_address_replacements','formato_direccion_de_facturacion', 1, 2 );

//Reordenamos los campos de la dirección predeterminada
function formato_direccion_localizacion( $campos ) {
    $campos['default'] = "{name}n{company}n{nif}n{address_1}n{address_2}n{city}n{state}n{postcode}n{country}n{phone}";
    $campos['ES'] = "{name}n{company}n{nif}n{address_1}n{address_2}n{postcode} {city}n{state}n{country}n{phone}";
	
	return $campos;
}
add_filter( 'woocommerce_localisation_address_formats','formato_direccion_localizacion' );

//Arreglamos el formulario de envío
function formulario_de_envio( $campos ) {
    $campos['shipping_email'] =array( 
        'label' => __( 'Email Address', 'woocommerce' ),
        'required' => false,
        'class' =>array( 'form-row-first' ),
        'validate' =>array( 'email' ),
	);
    $campos['shipping_phone'] =array( 
        'label' => __( 'Phone', 'woocommerce' ),
        'required' => true,
        'class' =>array( 'form-row-last' ),
        'clear' => true,
	);
    $campos['shipping_postcode'] =array( 
        'label' => __( 'Postcode / Zip', 'woocommerce' ),
        'placeholder' => __( 'Postcode / Zip', 'woocommerce' ),
        'required' => true,
        'class' =>array( 'form-row-wide', 'address-field' ),
        'clear' => true,
        'custom_attributes' =>array( 
            'autocomplete' => 'no'
        )
	);
	 
	return $campos;
}
add_filter( 'woocommerce_shipping_fields' , 'formulario_de_envio' );

//Arreglamos el formulario de cobro
function formulario_de_cobro( $campos ) {
	$campos['billing_postcode'] =array( 
        'label' => __( 'Postcode / Zip', 'woocommerce' ),
        'placeholder' => __( 'Postcode / Zip', 'woocommerce' ),
        'required' => true,
        'class' => array( 'form-row-wide', 'address-field' ),
        'clear' => true,
        'custom_attributes' => array( 
            'autocomplete' => 'no'
        )
	);
	
	return $campos;
}
add_filter( 'woocommerce_billing_fields' , 'formulario_de_cobro' );

//Añade el campo CIF/NIF a usuarios
function anade_campos_administracion_usuarios( $campos ) {
    $campos['billing']['fields']['billing_nif'] = array( 
            'label' => __( 'CIF/NIF', 'woocommerce' ),
            'description' => ''
	);

    $campos['shipping']['fields']['shipping_nif'] = array( 
            'label' => __( 'CIF/NIF', 'woocommerce' ),
            'description' => ''
	);
    $campos['shipping']['fields']['shipping_email'] = array( 
            'label' => __( 'Email', 'woocommerce' ),
            'description' => '',
       		'class' =>array( 'form-row-last' ),
	);
	$campos['shipping']['fields']['shipping_phone'] = array( 
            'label' => __( 'Telephone', 'woocommerce' ),