THere are a number of things that can cause hypertension in cats. It’s usually kidney disease or hyperthyroidism. I assume your vet has checked those. Heart disease, diabetes, adrenal tumors (pheochromocytoma), corticosteroid use, chronic pain or other inexplicable things can all cause it as well.
As to herbs, I wouldn’t combine them with meds on a kitty hypertension case. You’re dealing with pretty narrow safety margins on an animal that small and dropping her pressure with two things instead of one could be a bad idea.
Herb and Drug Interactions: General Principles
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.