I do not think you can have both : working permit and student permit. If you are on student permit you cannot just go and work anywhere you want without bothering to get proper working permit for working "off campus". The same story with studying when on working visa. You-on WP- can go and study "non-credit" courses or anything for your own fun and pleasure, but if you want to go to the university for some professional, vocational or academic certificate/diploma/degree you must apply for study permit , and because you cannot have both permits : working and student, you will have to"transfer" from working to student.
However, it case of topic starter if she wants to take a course or two to prepare for CRPNE I think it could be considered non-credit because she is not getting any diploma from the university. However, I would confirm that with CIC. You can call and ask them on phone without even telling them your name. You also can go to the university and ask them if they can to enroll you as a non-credit student for one or two courses.