To my knowledge, there is not a way to define reviewing interest terms or load a thesaurus into OJS. Reviewing interests are selected by whomever creates the user's profile with the role of reviewer, or later the reviewer can return to edit his or her profile to select certain reviewing interests. However, you could include this thesaurus in the Review Policy page (populated by text entered in the Setup section of the Journal Management pages), or you could link out to such a thesaurus created as as Static Page from the Review Policy or elsewhere. This wouldn't force reviewers to adhere to distinct reviewing interest terms by having them validated upon entry, but it would serve as a guide for them while selecting their reviewer interests.
If I misunderstood your question or my answer was not helpful, please feel free to reply with more information.
Electronic Publications Manager
Office of Scholarly Communication and Publishing
University Library System, University of Pittsburghvgabler@pitt.edu