I don’t know whether this is a common issue. From time to time, I find myself changing an event in my calendar (which may have originally been recurring) that is static and that I now want to become recurring. At least sometimes, Calendar will not let me change the event from static to recurring.

This problem forces me to manually copy all relevant information into a new recurring event. I find it frustrating.This may be because this is achieved transparently in Outlook at work.

Anyway, I found somewhat of a compromise. Duplicate the event (many ways including right-click on it then select Duplicate). Open the duplicate event, and set it to the desired recurring period. Of course, don’t forget to discard the original static event.

For some reason, whereas the original static event could not be made to recur, the duplicate static event can be.

This is not elegant by any standard even the lowest but is simple enough.

Source: Mac OSX Hints