What do you mean with 'external file'? The ORM package is an entire collection of files.
If you mean: can I install the package outside the docroot(s), then yes, I do that all the time. Just add the path where you install them to the package paths list in your app/config/config.php.
Note that they are processed in the order they are defined. I always place the shared one last, so I can override a centrally installed package with a local package of the same name (so I can test different versions for example).