It would be great if you could assign certain stock piles to certain work areas. Like assigning a crop field to a single stock pile making it so nothing else could be put there and the farmers wouldn’t put the food anywhere else but that stock pile.
May happen in the future, but in the mean time have you considered just making a food-only stockpile somewhere, and forbidding food from other stockpiles? So long as the food-only one is closest to the farms, the food will always be dropped off there .
That’s a good solution for now but I’ve noticed that the workers won’t always bring it to the nearest stock pile.