How can I model this object hierarchy in Fluent NHibernate without violating DDD principles?

I don't think Reviewer should inherit from User in the scenario you've described. I would have the Reviewer class hold a User object instead (composition over inheritance).

Sounds like a the Reviewer is not really modelling a person, but modelling a role or assignment the User takes on. I'd say your domain model is flawed in this aspect. Tweak Reviewer to be an association class between a User and a Contract.

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