Unfortunately, it is not possible to add anything after your order has been placed.
Did you forget to add something to your cart? In that case, we advise you to try and cancel your order via our chatbot which can be located on our homepage.
When our support team confirms your cancellation, you can simply place a new order. Be advised that this has to happen as soon as possible after placing your order, as orders can no longer be cancelled once they entered the processing stage.
Another option would be to simply place a second order for the missing items.