Your sentence is not correct. We say "Bon appetit" just as we serve food to someone. A waiter might say it at the same time that they put your plate of food in front of you. It wouldn't make sense to add "if you haven't already eaten". It means "Enjoy your meal" (note that that is not a direct translation. That would be "Good appetite", which makes no sense in English).
Note that I have changed your thread title. Titles must include some or all of the words you are querying.