Incorrect.
Not economically, there isn’t. You would have to redesign the entire airframe, or at the very least modify it so heavily that you would need to recertify it all over again.
Doing a band-aid fix would also increase the weight and lower the performance of the aircraft. Take the F-16 as an example. Just to get radar stealth, not even IR stealth, you would have to swaddle the aircraft’s internals with radar absorbent materials, you would have to redesign the composite skin of the aircraft for radar absorbent materials including the air intakes, you would have to somehow come up with an internal weapons bay, and at the very least redesign or relocate almost all of the avionics to make room for the RAM and bay. And when you’re done, you’re still going to have an inferior aircraft to the F-35.
How do I know? I asked a Lockheed engineer friend of mine (from the F-35 project) about retrofitting the F-16, and he just laughed at me. Then he explained the same thing and saidthat it would be cheaper to just go ahead with F-35 production.