I would like to adopt an outdoor cat, but my herding dog is aggressive toward cats on our walks. I am not sure if he will get along with one as a family member. I don't want to bring an adopted cat into a hostile environment.
As far as I know, there is no other possibility to train your dog to accept cats than by having a cat. But some requirements should be fulfilled that it is less stressfull:
Your dog obeys perfectly. You can handle him without a leash and are able to let him sit down, even if there is a cat.
You have enough space to separate both at the beginning, possibly for some weeks.
The cat should not be afraid of humans and especially not of you. Because there would be too much problems at once to handle.
You should be honest with yourself and be able to stop the training if you see that there is too much stress for one of the animals and no progress.
If all that is fine, you can start:
Take this cat to your house into a separate room. The cat should have food, drink and a litter box available and something to climb and hide. Even if the cat is an outdoor cat, you should keep the cat inside for at least some days, until she accepts your house as home.
After one or two days start with the training. Let your dog sit and let the cat come out of the room. Let the dog watch the cat, but hinder him from hunting the cat. Stay calm, don't shout at the dog, even if he is growling. After a while start to keep your dog busy. Train some tricks he already knows, give him treats. Try to ignore the cat. Don't play movement games, it could stimulate your dog and frighten the cat. Play calm games. The dog will have problems to give you attention. That is normal. If he is doing good, or just a little bit better than at the beginning, praise him, give him treats. Than stop the training. Do so by let the dog sit down, go to the cat, take her and put her back into her room. Then let the dog walk freely wherever he wants to go. He will run to the door where the cat is behind and sniffing at the door. That is ok. Let him do that. If he starts to scratch, say "no" and call him. If he is sniffing fanatically stop him, too. After some minutes do something very active with your dog. Go on a long walk or play an active and rough game. Give him the chance to burn off energy. He will have produced much adrenaline, and he should be able to get rid of it. When you come back, he will probably start sniffing immediately. Stop him doing that, let him sit down and cuddle him.
Retrain step 2 the next days. Be aware that the dog won't relax much the first days, even if the cat is at the other room. Try not to make a big deal out of it. Stay professional. If you get the attention of your dog more easily, increase the time the cat is in the same room as the dog. You can have multiple training sessions at one day, but don't overwhelm them.
Start to interact with the cat in front of the dog. The best is, if there are at least two humans. One is playing with the cat, another one with the dog. Don't play movement games with the cat, it would animate the dog to hunt. Start with cuddling the cat.
Don't force the cat and the dog being very close together or sniffing at each other. At first hinder them to get closer than one or two meters. When the dog is able to relax and nearly ignores the cat being in the same room, you can let them come closer. If the dog has a favorite place, always hinder the cat from going there and never let the cat eating the dog food. You don't want the dog being jealous. But don't allow it the other way round either.
Be prepared that it is a long way. You have to find a solution for the time when the cat is ready to get outside, but the dog doesn't like the cat, yet. Avoid every fight between them. Rather it takes longer as it could, than forcing a step and destroy every progress they made so far.
Probably your dog has never seen you holding and treating a cat with a nice attitude. I had a dog in the who had the same attitude when I adopt him at first time. After he saw me holding my cat and petting him over and over a number of times, while keeping a distance between them, my dog started to consider it as a member of our pack!
Same thing goes with my cat. When he saw me petting the dog he started feeling safe toward him.
(note : always keep a distance) I am not saying this method is going to work with your dog 100%. Don't trust him with your cat yet. I'm just telling you my experience that may be helpful to you.
This thing has to be coupled with monitoring and attendance for couple months before you can let them with each other alone.
And in the end this depend on your dog's aggression level.
I'm not an animal expert but that was just my experience. I hope this is helpful for you.
Have a good day :)