Extending the shape item in powerpoint

In Powerpoint in Office 2007 using Visual Studio 2010 and Tools for Office I want to extend the shape item to include an x and y co-ordinate of the centre of the shape, (ie left, plus half the wide, top plus half the depth).

I want to modify the shape to store and maintain that value.

So I wanted to extend the shape class, but when I try it I get the message classes can only inherit from other classes.

Turns out the shape is not a class?????

Code:

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Imports Microsoft.Office.Interop.Powerpoint
Public Class MyShape
    Inherits shape

End Class

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Doesn't work, what should the structure be, or is it even possible??