To have radiobuttons like those on the image you should create a custom skin and to edit the look and feel of the radio buttons sprite image. This is a tutorial how to create your own custom skin for RadFormDeocrator
I am also attaching for your convenience the original radio button sprite image from the Default skin and the same image after small changing of the look and feel of the checked radio buttons states to have better visual explanation how you should edit the original sprite to achieve the radio buttons that you like.
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