What I would suggest you is to change the ControlTemplate of the RadWindow control. Please, find the attached file - it contains a project that shows how to change the ControlTemplate of the RadWindow control and sets the ControlTemplate to be the default one of the Office_Black theme. The buttons come after the <!-- Window buttons --> commnet.
Hope this helps.
Sincerely yours,
Miroslav Nedyalkov
the Telerik team