Q: Are there special preparations I should make if I want to have a broody hen adopt and raise my shipped chicks?
A: There is little more beautiful than a mother hen sheltering chicks beneath her wings.
And there is little more efficient! If you have the luxury of having a broody hen to raise your chicks, you should totally take it. But there are some important things to be aware of!
When you are having a broody hen raise new baby chicks that have been shipped to you, first remember that your hen has to be currently broody. If she is not broody when you try to introduce chicks to her, she may try to kill them, but she certainly won't adopt them! Just having a hen from a breed that often goes broody (like silkies, orpringtons, cochins, etc.)--that is not enough. Your hen must be brooding at the time you introduce the chicks to her.
Second, presuming your hen is currently broody, we recommend allowing your new chicks six or so hours in a heated (~95F) brooding area first, with access to chick feed and water. Shipped chicks will need a little time to recover, and momma hen may not instinctively know what they need. She has no way to understand the journey they have just been through.
The reason for that is this: when hens hatch fertile eggs on their own, they hover and continue to set for a while before taking the babies to eat and drink. In fact, their natural instinct has them wait for about 48 hours before they cease egg sitting mode and begin chick rearing mode, moving from the nest. The chicks absorb the egg yolk into their abdomen right before they hatch, so the chicks have a reserve that means they don't have to eat and drink right away while they are waiting. The reason it takes a while for hens to break out of the egg sitting trance is that the chicks need time to dry off beneath her, and all her eggs need sufficient time to complete hatching. She can't jump up after the first one has hatched! That would doom the rest of her clutch. She must wait until they are all done, so her instinct is to stay on the nest about 48 hours.
However, with babies who have shipped to you, it is important to give them a little time to eat and drink first, before introducing them to momma hen. Shipped chicks may not arrive at your home for a day or two (depending on your location relative to your chosen hatchery), so your chicks will have already taxed their reserve. However momma hen won't know that. She will think they just hatched, and she most likely will not know they need to eat and drink right away. She treats them as her own, as if she had just hatched them, but shipped chicks have different needs in those first few hours, because by the time they arrive at your home, they will need to eat and drink!
So, when using a broody to raise your shipped chicks, definitely give your babies a little time, six hours or so, to recover in a brooder from shipping before introducing them to your hen. If you are buying young chicks from a feed store (or if momma hatches them herself), they will have already had time to eat and drink and recover, but chicks that ship directly to you just need a little time. This way they can eat and drink and revive from the stress of the trip, then will be ready to settle down beneath mom. They will be sated and will have time to wait until mother hen's instincts to leave the nest kick in, as many as 48 hours after she first feels the babies beneath her. Aside from this delay in introduction, once they are with momma hen, all you will need to do is provide quality chick feed and water in a safe place for brooding.
You will want to give them a safe place apart from the rest of the flock, because sometimes the other hens might try to hurt the babies (your other hens can think the babies are flock invaders!). Although momma will try to protect them, she is not always successful. Your new brood of chicks won't even need a heat lamp once they are with their mother hen, because the hen will keep them warm for you. Having your chicks raised by a broody insures they are very healthy and happy, and also that they can integrate into your flock much more easily when it is time, since momma hen helps teach them about the pecking order in your flock, and about appropriate social behavior.
Some people think that chicks raised by a broody are not as friendly, but that is not necessarily true. If your broody hen is very friendly to you, she will teach her chicks to be friendly, too. Some of my friendliest chickens were raised by a snuggly silkie momma, who taught them to rush over when they spotted me to see if I had any treats, and to jump up on my hands to be petted and loved.