Output the SKU

Hi,
When products are added to the basket, they contain the SKU, and whatever happens to these order afterwards, the SKU is included. However, the Free Gift(s) do not have a SKU listed, in our situation, this causes a problem when exporting orders via XML as the SKU is simply blank on the free gift and we are not able to deduct stock correctly outside of WooCommerce due to this.
Anybody know how to do this? I’ve tried trolling through the code but can’t make head or tail of it!