I finally found a solution after studying ask.php file and qa-theme-base.php.
public function form_field($field, $style)
{
if (@$field['tags'] == 'name="extra"') {
$this->output('<input type="radio" name="extra" value="" checked> Empty Value <input type="radio" id="extra" name="extra" value="ABC"> Value ABC <input type="radio" id="extra" name="extra" value="XYZ"> Value XYZ');
}
else
qa_html_theme_base::form_field($field, $style);
}
However, it will never be perfect, because when the user re-edits his question, the normal text form reappears. This is quite disturbing!
What is the function or php file that determines the layout of "question edit"? 
