IMO, it's not possible to foster an older chick to a hen. Ideally, it needs to happen within 48 hours. This is when the chicks were created to imprint to the hen. From a natural instinct perspective, this makes oh so very much sense: The chick has enough yolk reserve to last 48 hours. During this time, Mama keeps the chicks tucked under her, and sits tight to the nest, allowing late hatchers to join the brood. When all have hatched, the babies are ready to eat, and Mama comes off the nest. What's the first moving object they see? It's Mama, calling them to their first meal. So, they come to her, and imprint on her. The entire brood typically is between 0 and 48 hours of age. An older chick is past the age of imprinting. So, even if Mama wants to brood an older chick, the older chick may not be responsive to her calls.