I just raised this questions to support. Will report back if I got an answer.
Cheers, Robert
Update: After only 40 minutes I already got an anwser
(awsome response time for the support!!):
Unfortunately it is not possible to mutate an existing and paid for order.
If you would like to have several items in one order, you can have us cancel and refund your existing order so you can place a new one including all the items.
This does mean that you shall have a later delivery time for this new order than the old, existing order.