Along these lines . . . Crushed red pepper? I have used that in birdseed and homemade suet blocks to discourage squirrels. I am not sure if the squirrels would just eat around it when it is in chicken feed, but you could give it a try.You can also add some of these natural repellants. . .
It would probably be more effective in a mash or fermented feed, where the liquid would spread the heat throughout the food. But then you would have to change your feeder.