I managed to solve this one. I created another BO called "CalEntry" with attributes "Date" and "Subject". I then created a process with the rule:
CREATE Calendar WITH Calendar.StartTime=CalEntry.Date,Calendar.EndTime=CalEntry.Date,Calendar.Subject=CalEntry.Subject
I then start this process from the calendar through a custom button. Unfortunately with this solution, users can´t add an appointment to a date by clicking on that date in the calendar. Not the prettiest solution but it works for now and I might try to tweak this further in the future.
NB: Some Calendar attributes e.g. are set to initial value ("All Day" e.g. set to "Yes").