What works for me is ssh-ing to ZeroShell and then go to the ZeroShell system prompt. From there you can scp to/from anywhere in your network that has password authentication.
And, if you want key authentication, just scp the keys to ZeroShell.
This is what I use to backup ZeroShell to a local server. And what I used to bring Joe to ZeroShell.
Admittedly, mine is a Linux-only network, but from what I have heard from the Mac, it should be a piece o’cake.