Out of the box, you can't.
I use a 'loader' snippet for that. Create a snippet, name it FileLoader (or anything you please)
Insert:
<?php
require("assets/snippets/".$name."/index.php");
?>
Where name is the name of the folder your snippet is in. (e.g. assets/snippets/mysnippet/index.php)
Now you can put the contents of that snippet in an index.php file in that folder and you can access by ftp and edit in your favorite editor.
change the path if you wish to something that suits you.
And wherever you need the template or snippet you can make a call to it:
[[FileLoader? &name=`mysnippet.php`]]