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Hi, Welcome to Just Answer. I'm Chris, a small animal Vet based in the UK. I am reviewing your question and will get back to you as soon as possible
Thanks for taking the time to look after this stay cat, it is a shame that she is aggressive but this is often the case with feral cats that have not been used to human contact at an early age. There isn't anything that could safely be added to the food oral sedatives are generally pretty unreliable in cats. I would recommend contacting your local brach of the Cats Protection League, they may be able to lend you some thing called a cat trap - basically a cage which you put food into, when the cat enters the door closes behind them. I would recommend having the cat checked for a microchip and neutered as well if not already done. I hope this helps, please let me know if you have any questions or need further advice.