$node->webform[additional_emails] in webform 3.x

You can add custom e-mails to a Webform by added onto $node->webform['emails'] in hook_node_load(), though that could have some odd side-effects since users would be able to edit the configuration of those e-mails in the UI. So if you only want those e-mails to exist at the time an submission is sent, you'd be better off adding on those e-mails through hook_form_alter() on the webform_client_form and modifying the $form['#node'].

On a further note, the submission guidelines state that "how do I code..." questions are not supported in the Webform issue queue, so you'll probably need to take it from here. Though any solution you find would be great to post here for others to find.