Additional Information:
The reason you cannot add user-defined (new) grouping attributes to use in the Cost Plans is due to the design as noted.
The Grouping attributes are tied to the Matrix Columns.
So of the 15 attributes in the Matrix Columns, there are 11 attributes that are used directly in the Cost Plan Grouping :
For Planned Values: Charge Code, Cost Type, Department, Input Type Code, Location, Resource, Resource Class, Role, Transaction Class
For Actual Values: User Value 1, User Value 2
The other 4 attributes available on the Matrix Columns configuration page, are at the project level and thus not really needed for Cost Plan Grouping:
Client, Project, Investment Class, WIP Class