Hi Jorge Pascual,
There is a general problem with some complex controls in Visual Studio design-time. These problematic controls have been disabled so that they can not be modified by inheritors. This issue appears for Microsoft controls and Telerik controls. You can have a look at this
blog. It refers to Visual Studio 2005 but the issue persists in 2008 also. You will have to make your modifications through code.
Best wishes,
Victor
the Telerik team