Its possible to pre-select any radio button by specifying a default value for the property on the actioon form associated with the radio button. The property setter method must be called from the constructor and the reset method. Note that it should not be called from the reset method if the bean is in session scope because the selections will be reset for each page.
Even if the form is in session scope, setting the value in the reset() method should be OK since Struts calls the reset() method prior to calling the setters on the bean. The reset() method will set it to the default value, and then setSelected() will set it to the value entered by the user.
If you don't want to set default values in the ActionForm, you can just set the value in the Action class that forwards to the JSP.