Yes, if your timetable allows. You should enrol normally on the regular subjects, and you may enrol on any other subject, from any study cycle, as an isolated subject. That enrolment involves the payment of additional tuition fees. The grade achieved will not count towards your PhD, except if you  request  a change of annual subject. However, students must take care not to take on too much in terms of work and time needed: the annual subject should correspond to the main focus of interest and probable area for the theme of a student’s dissertation.