Not sure exactly what you mean by "add one and select it", since it will be selected by default anyway. But, if you were to add more than one, it would make more sense. How about something like:
$('select').children().remove();
$('select').append('<option id="foo">foo</option>');
$('#foo').focus();
Response to "EDIT": Can you clarify what you mean by "This select box is populated by a set of radio buttons"? A <select><select> element cannot (legally) contain <input type="radio"><input type="radio"> elements.