add_action('woocommerce_checkout_process', 'myfunction');
function myfunction() {
$order = wc_create_order();
$order->add_product( wc_get_product($productid), 1); // This is an existing SIMPLE product
$order->set_address( $address, 'billing' );
$order->calculate_totals();
$order->update_status("Completed", 'Order status', TRUE);
}