The easiest way to convert your scenes from other renderers is using Corona Converter script by Martin “Deadclown” Geupel. This tool is bundled within Corona installer and is installed into 3dsMax 20XX/scripts/CoronaRenderer folder. It converts most materials, maps, lights, and even proxies from Vray, Mental Ray, Final Render, and other renderers with a single click. Usually the result is very close to the original non-Corona rendering, although sometimes additional manual tweaking of materials or lights may be necessary.
Note: the converter will only work if you have both Corona and the other renderer installed on the machine where you are using it. It is fine to use the demo or expired trial license of the other renderer.