Yes, you can. When you preorder you go to a short line with registers and the CM will double check your order. You can make alterations at that time before paying.
Right, but the OP is asking if they can order the desserts AFTER they have eaten.
The title of the post asks if dessert can be added after they get there (not after they've eaten). Although I see now that the wording in the first post seems like it's after they've eaten. I admit I just answered the question in the title when I replied, rather than reading the actual post.