You can get the editor array and then add a value to it and set it again if editor is not the only array in the ParameterBag:
$data = $this->getRequest()->request->get('editor');
$data['password'] = 'string';
$this->getRequest()->request->set('editor', $data);
Edit
Looks like there is a question in a google group that is similar with a similar answer: https://groups.google.com/forum/?fromgroups=#!topic/symfony-devs/2-SWFtFKwxQ