Specifying the columns for listgrid in the DMI

Hi

I am currently trying to populate a listgrid with data that I obtain from a webservice. I do not wish to invoke the webservice directly from the client, since I have some validation and processing that I need to perform on the data before I load it into the grid.
The webservice returns to me the data as well as the columns and their title. I am not aware of the the columns and their title until I receive the response from the webservice.

My current approach is that I have created a CustomDataSource. In the executeFetch method I have my logic to return my list of records. Right now I have hard coded the columns in my ds.xml because of which, if my recordList contains additional columns they do not get added to the grid..

Is there a way I can do something like datasource.setFields or somehow dynamically change my fields on the server?Or is there any other approach to this?

Thanks