Hi.
Currently the labor rates functionality does not allow you to add user-defined columns or attributes. Off the op of my head, have you considered creating multiple roles for each employee level? For example, "Director - Level A" and "Director - Level B". This should allow you to then apply different incentive plan rates for each. The downside would be that you will need to maintain two (or more) of each role. In some organizations it may not be an issue but if you have a very deep org title structure then this may not be the best option.
I'll see if I can think of some other way to provide the flexibility you're looking for.