Light is/produces a electromagnetic wave, but is the opposite also true? A similar question has been asked, and to some extent the answer is: you need an antenna with $L \approx 200nm$. But it's not entirely clear to me if this is completely hypothetical, as "Marek" also says:

Yes, if you heat it :) But assuming you want to use it in the usual way the answer is no.

How does the creation of X-ray waves and Microwaves differ from the creation of Radiowaves? (if it does) 
Would (and I don't know to what extent this is hypothetical) the creation of electromagnetic waves with a wavelength between $400nm$ and $700nm$ (same visible light) actually produce light? (i guess as in: 'would these field disturbances yield photons?')
 A: Electromagnetic (EM) radiation between 400nm and 700nm in wavelength is the same thing as light. There is no difference. Neither is there a distinct difference between light, ultraviolet, x-rays, gamma rays, infrared, microwaves, or radio waves. Those names are all just human convention to specify EM radiation in certain frequency ranges. And regardless whether it's light or x rays or radio waves, they're all made up of photons.
Even though it's all fundamentality EM radiation, in practice, the way we interact with different types of radiation is very different. At radio frequencies/low microwave frequencies, conventional electronics works well for interacting with EM radiation. That means the microwave signal is generated with something like a resonator, and transmitted into free space with an antenna. At optical frequencies, conventional electronics fails, and thermal or quantum mechanisms for generating light are more typical (e.g incandescent bulbs or lasers). At x-ray frequencies, it's more typical to accelerate electrons more directly to generate the radiation (e.g x-ray tubes or syncotrons).
