Setting a Config Item

Hello
Is it possible to set a value into an item loaded from a config file?

I read into the manual the page about "Config Class"
Expecially about "set_item" function

But which is the syntax for 'item_name' in case item is loaded from a config file?

Example:

In my controller:

PHP Code:

    $this->load->library(array('ion_auth'));
...
    //get 'identity' value from loaded config file
    $id_type = $this->config->item('identity', 'ion_auth');
...
    $id_type = 'new_value';
...
    //set 'new_value' to 'identity' config item
    $this->config->set_item('identity', $id_type) ; 


In this case, I set 'new_value' into config array, but NOT in "ion_auth" occurrence...
Is it possible to do this?
Thanks