wc-customer-order-csv-export-add-weight-csv-import.php

function sv_add_weight_to_csv_export_import_format( $order_data, $order ) {

	$count = 1;

	// add line items
	foreach ( $order->get_items() as $item ) {

		$product = $order->get_product_from_item( $item );

		if ( ! is_object( $product ) ) {
			$product = new WC_Product( 0 );
		}

		if ( $weight = $product->get_weight() ) {
			$order_data[ "order_item_{$count}" ] .= '|weight: ' . $weight;
		}

		$count++;
	}

	return $order_data;
}
add_filter( 'wc_customer_order_csv_export_order_row', 'sv_add_weight_to_csv_export_import_format', 20, 2 );