Ofcourse you can, you just need to set up a port forward in the router pointing to the AppleTV IP.
This is however not recommended from a security perspective, since FTP sends the User/Pass in clear text when logging in.
The recommended way to do this is to use SSH/SCP instead. SSH is encrypting all data on the fly. Same thing here, for SSH you need to do a port forward in the router to the AppleTV IP.
To then transfer your data, instead of using an FTP-client, you use an SCP-client. For windows, you can use WinSCP, for mac you can use muCommander, for Linux you can use Gnome-commander.