form_validation.php in config and conditional validation rules

Here's a snippet of one of my sets of validation rules (currently in the body of the controller).

PHP Code:

$this->form_validation->set_rules('caseworker_contact', '<span>"Contact Case Worker"</span>', 'required'); 

if ($this->input->post('caseworker_contact') == 'yes') {
   $this->form_validation->set_rules('caseworker_name', '<span>"Case Worker Name"</span>', 'required');  
} 


It's that conditional rule that I am trying to figure out how to place in the config file.