If a course is not full, you can register via UC Extension (http://www.unex.berkeley.edu/). Here's what they have to say:

Through UC Berkeley Extension you may enroll in some regular UC Berkeley campus courses through an option called "concurrent enrollment." To be accepted in a concurrent course, all prerequisites for the course must be met. To enroll, you must obtain the consent of the instructor, the head of the campus department, and, in some cases, the dean of the department's school or college. Any other course requirements must also be met, such as a transcript of study. Enrollment must be for credit. You are considered an Extension student when enrolled through concurrent enrollment. Extension's Registrar maintains your records of study. Being accepted for a concurrent course does not constitute admission to UC Berkeley as a regular student.

It costs nearly something like $750 to register for a regular 4 unit course. There are various restrictions... you can't take the place of a regularly enrolled Berkeley student, you cannot apply credit from graduate-level courses in Extension to a UC Graduate program, you don't get insurance, you don't get a library card, etc. There are ways around many of these restrictions (if you join the alumni association, you get library privileges, etc).