I was wondering if I could turn my Nikon 18-200mm lens into a Macro zoom lens by adding either a teleconverter or a macro-converter?
A macro-converter might work. It might not give you useful results. Kinda like trying to tow a bass-boat with a sports car. Just because you can get a trailer hitch rated for both, doesn't mean its a good fit.
For dewdrops and other subjects that do not move (or move very slowly) you can get probably away with something like this inexpensive macro-filter set You'll want to double check the filter size of your lens, as the 18-200mm Nikkor lens I referenced is a 72mm but yours might not be. This is also likely to be your cheapest solution.
For insects, the ring reversal method is likely to also be cheap but it requires setting the shot up, then flipping the lens over. This may prove too fiddly to be of a lot of use. This post goes into more depth about some of the more advanced options with ring reversal macro photography.
If your serious about this sort of photography you will eventually want to find an actual macro lens, like the smaller one in this set for best results.
I've had decent luck with my D3300 and that macro-filter set on the 18-55mm that came with the body in direct sunlight, so its certainly possible. Add a lens mounted ring-flash and you can worry less about direct sunlight.
I've not tried the reversal ring method.
Not a good idea. Teleconverters INCREASE the focal length and effectively extend the minimum close focus which is the opposite of what you want to do.
Best solutions are either a true macro lens or optionally a lens reversal ring used on a wide angle lens or a 50mm standard lens.
Google Nikon lens reversal ring. They are inexpensive. The lens reversal screws into the filter threads on the front of your lens so you have to get one with the correct thread size in mm. Then that lens is bayoneted BACKWARDS onto your camera. You will generally lose all electronic functions such as Autofocus and Auto exposure while using that ring. You will have to manual meter and manual focus (although moving the camera physically towards or away from the subject works best). Macro photos taken with this setup usually give extraordinary sharpness and resolution. That having been said, a true macro lens is by far the easiest solution,