not by the learner themselves. the Author role in Prime can create seminar (& webinar) modules as well as Activity Modules. The activity module can be anything like, as you indicated, a workshop, exercises, questionnaires and any other online learning activities.
So as long as the author created a module/course for this external activity the learner can add it to its training record.
PS: Prime also works together with external content providers through its API, like Lynda, Workday, Harvard ManageMentor, Get Abstract. more can be added through its API.