You can order anything you like, in fact during the 1st 3 nights on the 7 day cruises you can order anything on the menu from the other 2 restaurants, since I believe they share the same galley. For example I had souffles for dessert each night even though souffles were only on the Lumierre's menu. After the 3rd night the restaurants all have the same menu (e.g., Pirate's night).
Be aware that serving sizes tend to be large and that you will be provided with extra appetizers and desserts without ordering them. Kinda fun, but you'll end up with a ton of food on your table. Some of it is really good, some is merely ok.
Here's the rub though - you really can only order extra items up front. The servers follow the standard drink order/drink delivery/food order/appetizer delivery/entre delivery/dessert order/dessert delivery service process. If you really liked an appetizer and wish to have another order of it instead of dessert you might have a hard time getting it. I actually had better luck upstairs at the sit down dinner in Topsiders getting a la cart items than in the restaurants. It's not that the galley doesn't have them, it's just that the service process has the servers moving really fast in order to get everything done in a somewhat timely fashion (you'll notice this if/when you try to order an extra smoothie or alcoholic drink with dinner).