grouping nodes with collapsible fieldset
OK I found it.

In the HOOK_node_view I can do:

if ($node-&gt;type=='CONTENT_TYPE') {

  // clone from a fieldset nearby
  $node-&gt;content['#fieldgroups']['group_FOO']=clone $node-&gt;content['#fieldgroups']['group_ORIG'];

  // modify fieldset
  $node-&gt;content['#fieldgroups']['group_FOO']-&gt;label="xxxxxxxxxxxxxxxxxxxxxxxx";
  $node-&gt;content['#fieldgroups']['group_FOO']-&gt;weight=-1;
  $node-&gt;content['#fieldgroups']['group_FOO']-&gt;identifier=...
  $node-&gt;content['#fieldgroups']['group_FOO']-&gt;group_name=...

  // clone from a field nearby
  $node-&gt;content['field_FOO'] = $node-&gt;content['field_ORIG'];

  // modify field
  unset($node-&gt;content['field_FOO']['#items'][0]);
  unset($node-&gt;content['field_FOO']['#items'][1]);
  unset($node-&gt;content['field_FOO']['#items'][2]);

  // position fieldset and field
  $node-&gt;content['#group_children']['group_FOO'] = 'group_ORIG';
  $node-&gt;content['#group_children']['field_FOO'] = 'group_FOO';

}