I don't think you can do this on the Report Definition itself. I guess you could try to create two different UI presenting the expected number of columns and jump from one to another with a WHEN condition maybe.
Hi Deva, Could you explain more on your use-case? When you create a report definition you will be having columns already defined so I think you meant while presenting the columns (either with repeating grid or repeating dynamic layout) you want to mask few columns. Right?