How to extend SPGroup to add custom fields?

I have a Sharepoint farm with many SPGroups for employees and many other groups for managers.

Eg. EmployeesLocal, EmployeesInternational, ManagersTechnical, ManagersHR etc.

I want to check if a user belongs to an employees group but I don't want the overhead of checking against each group as this will be a maintenance headache. Is there a way to add a custom field or property to the SPGroup class and then set it to "Employees" for all the employee groups and similarly for managers.

Also, if there is a way to create a hierarchy of SPGroups then I could add a parent Employee group and other groups under it. But I believe this is not possible. I would appreciate if someone can confirm this.

How would one go about solving such a problem.