I'm using Natural Hairball Aid by NaturVet.
You don't add it to food. I don't know of any hairball remedy you
can add to food.
With the gels, like VetBasis (hard to find), and NaturVet, you put a
1-inch ribbon on a plate and let them lick it up. Some cats will.
If cat won't eat willingly, you will have to finger-feed.
Put a dab on your finger and place that in the roof of cat's mouth,
just behind front teeth. Release cat and let him swallow naturally.
I can usually get the dose into Tommy in one or two globs.
I give NaturVet every 3-4 days for both my cats...I can always tell that
it's working because the next day their poopies are full of hair.
I have not found NaturVet in big box stores like Petco nor PetSmart. I get
it at a pet speciality store...sort of a health foods store for pets.
Health Foods stores might have it, or Whole Foods grocery or Trader Joe's.
Their web-site is
www.naturvet.com and phone number is 1-888-628-8783.