EKReminder(subclass of EKCalendarItem : EKObject : NSObject just like EKEvent) is not meant to be subclassed. Event Kit objects are used to represent database records. Creating a subclass of EKReminder will not magically insert new fields in the Event Kit database, nor will casting a EKReminder to something else magically change that object' class.
The only way to store extra fields into the database is to have a direct access to that database, which Apple reserves for themselves.
As you cannot add new fields to the Event Kit database, you can either use the existing fields (for example, add the event ID in the notes of the event) or extend it with a second database managed by your application (in which case you can't use calDAV to store your objects).
Found the answer here and modified it to fit my question. Properly subclassing the EKEvent Class