I'm able to get my custom module-provided fieldgroups to show up in the manage fields section with Rel, but they don't seem to work with Multistep (which is pretty understandable).

I currently have a hacky way of putting custom module-provided node fieldsets I want into a specific step, maybe it will help Rel or someone else.

  // Our crappy way  of merging fields into fieldgroups because "rel" doesn't
  // quite work with multistep.
if ($_GET['step'] == 'some_step') {
  foreach (array('module_group1', 'module_group2') as $field) {
    $form[$field]['#group'] = 'group_that_is_vtabs';
    $form[$field]['#access'] = TRUE;
    $form['#group_children'][$field] = 'group_that_is_vtabs';
    $form['#groups']['group_that_is_vtabs']->children[] = $field;
  }

  // needed, else fields get duped as well
  unset($form['additional_settings']);
}