short version, no. The system does not give you an entree credit/dessert credit/drink credit...it's a meal credit. so you either take all 3 allowed components, or not. But the meal components cannot be divided up, it's a use-it-or-lose-it scenario. About the best you can do is try to choose a dessert that can be taken to-go with you & eaten later.
Now, if you're eating at your resort counter service and ice cream is an option, most resort places will mark your receipt so that you can come back 30-90 minutes later for the ice cream (POFQ told us 30 minutes, but I've seen reports from others that some resorts allowed a full 1.5 hours to come back later for the ice cream) They allow the delay because if you had to take the ice cream at the same time as the rest of your meal it would be melted, of course. And you had to come back to the same resort counter service to get your ice cream dessert...you couldn't take the receipt to a park ice cream counter.
