Yes I wouldn't mix it in right now. Keep them for when they are a little older. Keep them dry and cool, such as in a closed container that is airtight in the garage up off the floor.
You could even freeze the pellets but take care when defrosting and only defrost what they will eat immediately as moisture will be there after they have been frozen. Don't thaw and let them sit. If kept properly, chicken feed should only be stored for at most 3-4 months. Frozen of course it will last longer than that. You don't want the vitamins to degrade or you will end up with deficiencies. Make sure they have greens at all times too of course, like grass clippings or veg.
