One of the most important tasks of a librarian is the assignment of appropriate subject(s) to a resource within a library's collection. The subjects usually belong to a controlled vocabulary that is specifically designed for such a task. The most widely adopted controlled vocabulary across libraries around the world is the Library of Congress Subject Headings (LCSH). However, there seems to be a shifting from traditional LCSH to modern thesauri. In this paper, a methodology is proposed, capable of incorporating thesauri into existing LCSH-based Information Retrieval-IR systems. In order to achieve this, a mapping methodology is proposed capable of providing a common structure consisting of terms belonging to LCSH and/or a thesaurus. The structure is modeled as a Simple Knowledge Organization System (SKOS) ontology, which can be employed by appropriate subject-based IR systems. As a proof of concept, the proposed methodology is applied to the DSpace-based University of Piraeus digital library.