Add a product to an order in WooCommerce programatically

woocommerce-add-product-to-order.php

<?php
// Get order data.
$order = wc_get_order( 1246 );

// Order total.
$total = $order->get_total();

// Add Product data.
$product_id = 8;
$quantity   = 1;
$args       = '';

// Add Product to order.
$order->add_product( wc_get_product( $product_id ), $quantity, $args );

// Get product details.
$product = wc_get_product( $product_id );

// Product price.
$product_price = $product->get_price();

// New order total.
$new_total = ( $product_price * $quantity ) + $total;

// Set new order total.
$order->set_total( $new_total );

// Save order.
$order->save();