OJS does support multiple roles. If you're a Journal Manager, you can add a new role to anyone's existing account by using the "Enroll Existing User" link at the bottom of any role's list in Journal Management. Alternately, the user can self-enroll for an additional role by logging out, clicking "Register", following the "Click here if you are already registered with this or another journal on this site", and filling in the form.
The next release will also include the ability to add one of the available self-registration roles (e.g. Reader, Author, and/or Reviewer) on their profile page.
Open Journal Systems Team
Don't miss the First International PKP Scholarly Publishing Conference
July 11 - 13, 2007, Vancouver, BC, Canada