Well you could give them the teacher role in the subject areas they look after - this will then enable them to post up relevant info and resources. If you don't want them to be able to look at grades etc you would have to create a new role "subject librarian" based on the teacher role and remove some of the teachers' permissions.
Or if by post you mean you want them merely to post information and not upload stuff then you can just make them students and the teachers in the course can set up a forum which the Subject librarians can use to impart information.
Roles and permissions
Subject Librarian role?
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