For cathodic protection of the underside of oil tanks, it may or may not be feasible. One question is whether there's an existing liner under the tank bottom that would insulate it from the ground. If you have to replace the bottom in an existing tank, a cathodic system can be added at that time.
For cathodic protection of the interior of water storage tanks, it is not uncommon to add a system to an existing tank- contact the cathodic suppliers. Easier to do this when the tank is down for repainting or other repair.