Form Field Containers
Drupal uses renderable arrays, so there is probably something like this:

'location_outter' =&gt; array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; 'location',
  'location_inner' =&gt; array(
     '#type' =&gt; 'fieldset',
     '#title' =&gt; 'location',
     'city' =&gt; array(
        '#type' =&gt; 'textfield',
        '#title' =&gt; 'City',
     ),
   ),
);


So i think you can, if it is form, try use alter hook, for example yourmodule_user_form_alter and modify that renderable array.