Botón WooCommerce en una ventana nueva

En functions.php de tu tema hijo:

remove_action( 'woocommerce_external_add_to_cart', 'woocommerce_external_add_to_cart', 30 );
add_action( 'woocommerce_external_add_to_cart', 'rei_external_add_to_cart', 30 );
function rei_external_add_to_cart(){
    global $product;
    if ( ! $product->add_to_cart_url() ) {
        return;
    }
    $product_url = $product->add_to_cart_url();
    $button_text = $product->single_add_to_cart_text();
   do_action( 'woocommerce_before_add_to_cart_button' ); ?>
    <p class="cart">
        <a href="<?php echo esc_url( $product_url ); ?>" target="_blank" rel="nofollow" class="single_add_to_cart_button button alt"><?php echo esc_html( $button_text ); ?></a>
    </p>
    <?php do_action( 'woocommerce_after_add_to_cart_button' );
}