Adopting a barn cat takes a bit of effort, especially if you are relocating the cat from a previous colony or area. Cats do better in pairs and it is best to adopt more than one at the same time so they can acclimate together. A male and female, or 2 females are the best pairs unless you know of 2 males that already get along together.
Cats need to be in an enclosed room for 4 to 6 weeks so they can get use to their new environment. This is very important. A cat will likely leave and attempt to find its former home and get lost or killed if they aren't contained for this period of time. If you don't have a room that they can't escape, then you can use an XL dog crate or 2 crates zip tied together so there is more room for the cat to move around. Drape a large sheet over the top of the crate so they feel more secure. Keep in mind that you will have to be able to clean and feed in this crate. Putting a carrier inside the crate is a good idea. This gives the cat a safe space to hide so the cat won't feel as threatened towards you while you are leaning inside the crate to clean and feed. Even putting a large piece of cardboard over the carrier door (just when you are cleaning and feeding) for the first week is a good idea so neither yourself or the cat feel stressed looking at each other. The first couple of days are stressful for the cat and they will try to escape. Once they realize that no one is trying to harm them, they usually calm down. If the cat does escape confinement before the 4 to 6 weeks, make sure you leave food and water out for it. Sprinkle its used litter around to help lure it back.
Daily feeding of kibble and soft food is required during confinement, as well as supplying fresh water. Enjoying appealing food in that confinement time will help them feel comfortable and will help to build a bond between the care giver and the cat. Try to feed at the same time every day (some owners feed twice a day as this is a good way for the cat to get to know you). Take the time to talk to them so they get use to your voice. People who make an effort to communicate with the cats have the most successful relocations. Over time it is possible for them to become friendly.
If the barn isn't heated, there must be insulated cat shelters. These are easy to make (there is lots of information on line to make these). It is important that straw is used inside these insulated cat shelters and NOT hay. Straw does not get moldy like hay does. A heat lamp is another good option for warmth. Having livestock in the barn during winter also helps to give off some heat.
Putting toys and scratch posts in the enclosed area will help to stimulate the cat during confinement period.
Ensure other animals on the farm are introduced slowly to the cats.
When the confinement time is over, open the crate door (or the room door) so the cats can explore the rest of the barn. Leave the area exactly as it is so if they get afraid they can run back to this safe area. It may take a few days to feel confident, but never force them out of the crate or room. You may only see them when it's feeding time, but in time they will relax and roam freely.
While the cats are allowed outside during the day, some owners like to bring the cats back inside at night so they are safe. Coyotes and foxes will kill them. This can be done with a routine evening feed inside the barn with the doors being closed behind the cats. The cats should always have a way to get inside in case there is danger. Installing a cat door can work for this.
By following the above guidelines, the acclimatization of your barn cats should be successful!