Rather silly question - extending core library

Hi everyone, 
I tho up to now I know codeigniter very well ... well - I don't. 
I have some CMS system build using CI2.0 + DataMapper (I know, I know - but I'm just employee) and trying to add some function to Upload library, so I can do:

PHP Code:

$this->upload->something($some_data); 


however adding in core folder MY_Upload that extends CI_Upload, throws an error. 
Is it possible in CI to extend Upload this way ?
Thanks - Matt.