There are some products on the market that you can try that have cat pheromones. We were dealing with a lot of hostility between our older cat Pogo and the new one, Lizzie, and calming collars have helped both of them relax. They give the animal a sense that they are in safe territory. I would leave a collar on her only when you can keep an eye on her because they're supposed to be worn loose around the neck, and the result is they can get their lower teeth under it and get stuck. This happened to one of my cats with a flea collar many years ago. We came home after a couple of hours away and found the poor kitty struggling. If she had been an outdoor cat in this situation who knows what might have happened!
Anyway, check with your vet before using the pheromone product, but my vet felt it was a good idea for helping Lizzie and Pogo adjust to each other. I bought the Sentry calming collars on Amazon, 3 in a package, and they're supposed to last about a month each, but I take them off when I can't be around, and store them in a sealed bag when they're not in use. Open this product over a trash can and not your kitchen table as I did. It has some white flaky material that immediately came off all over everything. They are available in pet supply stores, also, and there's at least one other brand, if you want to shop around.
There are mist devices, also, but a couple of the reviews said they shorted out and smoked (!) so I was leery of trying that. Also, they cover a limited area, whereas the collar goes wherever the cat goes.
Another item that we have tried, with a little temporary improvement in the cats' behavior was a paw paste. It's camomile and some other herbs in a chicken based gunk that you smear on their paws where they lick it off. The calming effect was good but wore off after a few hours, and Pogo jumped on our bed in the middle of the night and started a hissy fight with Lizzie - 2 nights in a row. That was a rude awakening, and that's when I decided to get the collars.
I don't know if they will make a feral cat into a lovey pet, but I'll bet it will make her calm down a bit and relieve some of the anxiety. It turns our meanie into a Stepford cat. We found out that Lizzie, from a local shelter, had a bad case of ear mites, and we think that possibly Pogo could smell something wrong and didn't want her around for that reason. We are keeping them in separate rooms until we have the follow-up with the vet confirming no more mites. Pogo has always had at least one cat buddy except for the last few months since we lost our beautiful KittyBee.