Delphi XE7: How to correctly derive from a class+interface pair? (XSuperObject: ISuperArray+TSuperArray)

Question

I need to add some functionality to the TObjectArray class in the XSuperObject library. I could just add it directly into the source code in XSuperObject.pas, but that's not how I should do it. Instead I want to derive my own interface+class from the ISuperArray/TSuperArray pair. But how? I know how to derive a class on its own, but I've never done it before when interfaces are involved.

XSuperObject.pas defines these classes as follows:

type
  ISuperArray = interface(IBaseJSON<IJSONArray, Integer>)
  ['{41A2D578-CFAB-4924-8F15-0D0227F35412}']
    ...
  end;

  TSuperArray = class(TBaseJSON<IJSONArray, Integer>, ISuperArray)
    ...
  end;

What I would like to do is this:

uses
  ...
  xsuperobject;

type
  TProductJson = class(TSuperArray)
    CategoryIndex: integer;
    SubcategoryIndex: integer;
    constructor Create(JSON: String);
    procedure AddCategory(Title: string);
    procedure AddSubcategory(Title: string);
  end;

Or should I also derive from the interface at the same time? Maybe something like this?

uses
  ...
  xsuperobject;

type
  IProductJson = interface(ISuperArray)
  end;

  TProductJson = class(TSuperArray, IProductJson)
    CategoryIndex: integer;
    SubcategoryIndex: integer;
    constructor Create(JSON: String);
    procedure AddCategory(Title: string);
    procedure AddSubcategory(Title: string);
  end;

So my question is: how do I properly derive my own class/interface from ISuperArray/TSuperArray?
And if I must derive from the interface as well, do I also need to duplicate the GUID, or create a new GUID, or drop the GUID?

And the second question: XSuperObject.pas defines 2 functions: SO and SA, to initialize a SuperObject resp. SuperArray. Will the SA function remain compatible with my derived interface/class? Or will it not be assignment compatible after I have derived from it?
(maybe a typecast could help there?)