Sadly no. The reason is our system won't allow us to alter any invoice once an order has been placed. Also the addition can then impact on the shipping as that is calculated on the weight of the items in your cart.
Any additions need to be placed on a new order.
Please include in the comments section the order number that you would like it added to and a daytime contact number in case we need to reach you.
If there is any saving to be made from the deliveries being combined then we will let you know and issue a credit to use on your next order.