Please see this article explaining how to use the Corona Converter Script:
How to use the Corona Converter? - 3ds Max
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 trial or even expired trial license of the other renderer.
For example - if you are converting V-Ray materials to Corona, you need to have both Corona and V-Ray installed on your computer.