I am sure mixing it with food will not matter. With all my animals, once they associate feeling better with the dropper , they calm down.
Also, if you are hesitant or nervous, the cat will sense it. If you are determined , they know they better be good :)
Do you have someone that can hold her...
Food can cause nausea in cats with renal disease. I use 200 mg of Slippery Elm herb twice a day in addition to Astor's Oil.
My 16 year old is now back to eating fairly normally and does not vomit. It is totally non-toxic for dogs and cats and works wonders.
good luck!