There is no built-in tool that can do that. The reason for a custom value to not be available is because if a value does not exist, then the element will not be decorated, as there is no CSS behind that class. Therefore, it is a must for the user to go to HTML, add the class and add the CSS that is related to that class.
As for creating a custom tool, I suggest you to rather create a custom dialog (http://demos.telerik.com/aspnet-ajax/editor/examples/customdialogs/defaultcs.aspx
) that enables the end-user to add custom CSS classes to the dropdown by adding the custom CSS rules that would be needed in the content area.
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