My suggestion, look at the existing attributes which can be used on Cost Plans for grouping including Department, Resource Class, Role.
Given that an existing attribute 'User Attribute 1' can't be used by 'Populate for Task Assignment', then creating a custom attribute also can't be used.
Alternatively, write your own process to populate the Cost Plan, based on your logic to populate the attribute, and do not use 'Populate from Task Assignment.