How do you add custom metadata to WooCommerce orders? Specifically a second external order ID

by

you can use woocommerce_checkout_update_order_meta hook to update order meta. Please check sample code

add_action('woocommerce_checkout_update_order_meta', 'custom_checkout_field_update_order_meta');

function custom_checkout_field_update_order_meta($order_id)
{
    session_start();
    if ($_SESSION['mapics_order'])
        update_post_meta($order_id, 'mapics_order', esc_attr(htmlspecialchars($_SESSION['mapics_order'])));
}