No, unaffiliated students do not need to attend a mandatory session.
Unaffiliated students may want to attend the sessions for the majors they are considering changing into. It's also important to meet regularly with your academic counselor to ensure you are making proper progress toward your major of choice. Remember that you need to declare a major by the time you attain 90 units. Look at www.due.uci.edu for change of major requirements.
Students that are interested in an engineering major might consider signing up for Engr 1: Introduction to Engineering. Engr 1 is a one unit course introducing students to the engineering fields and the profession.