I'm not an expert in Jasper Reports. However, I assume that if you want to add the columns at run time, you may have to post the requirements back to the server, recreate the necessary JSP (or other format) using the back end Jasper Engine and repost it to the browser. I doubt about the existence of smarter ways to manipulate the columns in the browser only, with out reposting to server.
- Winston
http://blogs.sun.com/winston