Hello Liu Peng,
Thank you for contacting us and for your question.
I am afraid that the fireCommand method does not support such a functionality.
The best way to achieve the implementation you are considering is to use the AjaxManager ajaxRequest client method and handler your group by expressions on the server.
For your convenience, I am sending you a small sample demonstrating how to achieve that.
Please, use it for comparison with your implementation and let me know if I can help you further.
All the best,
the Telerik team