Hello Balaji,
Apologies for delay in response.
In order to achieve your requirement you need to manually add the node row and then create the grouping based on that.
Kindly check the below documentation URL detailing how to go about the same :
http://helpcentral.componentone.com/nethelp/c1flexgrid/#!Documents/creatingoutlinesandt.htm
I will try to create a sample application and will update you as soon as possible so that you may use that as reference.
Warm Regards,
Rijo