I was wondering if there is a way to use the Alias directive with just one file. Or there was a hack to do this, without having to go into another directory.
Yes you can - the config for the
Alias directive says:
Syntax: Alias URL-path file-path|directory-path
So you can have a directive such as:
Alias /robots.txt /path/to/robots.txt
You could use mod_rewrite. Here is a good tutorial explaining how it works and how to use it: