Descendant of TPlannerItem

I was hoping it would help me:

  TjstPlannerItem = class(TPlannerItem)
    FSimpleShape: TSimpleShape;
    procedure SetSimpleShape(const value: TSimpleShape);
    constructor Create(Collection: TCollection); override;
    property SimpleShape: TSimpleShape read FSimpleShape write SetSimpleShape default ssCircle;

But in use, I (of course) came across a mismatch of function declarations. In the manual I did not find a way to extend the properties and others for TPlannerItem. Is there any way?
I want to use it in TjstSimpleItemEditor = class (TCustomItemEditor)

I solved the task via Item.Tag. But I'm still wondering if the offspring can be easily used.

Demo 22 shows how you can descend from TPlannerItem to extend it

See: "Demo22 Demo showing a planner with a custom TPlannerItem class" at