FSG Report- How to Group Couple of Departments and Report the Totals

Requirement as follows,

In our COA, we have around 100+ Departments and some of them are grouped under a parent department. Recently we developed a Departmental Cost Report where in the cost is shown by Department Wise where as business expects some of the departments should be grouped (either using a existing parent/child relationship) or adhoc grouping only for this report.

Is there any way to group the departments and generate the FSG at parent level or at higher node level?

Appreciate your inputs .

Regard

Uday