How to add field in an existing fieldgroup by code

Hi,
I use fieldgroup for generating tabs when a node is displayed.
I need to add external field (from another content type) in the tabs generated.
So I created a module and add :
function addingroup_field_group_build_pre_render_alter(& $element) {
drupal_set_message($element['#groups']['group_generalites']->children['0'],'error');
}
a message appears on the node display with the name of one present field.
I need to know how can I add in the $element['#groups']['group_generalites'] a new children!
Best Regards,