Is it 'hacked' to run XBMC? If so, yes, definitely, can connect via Samba.
I have my iTunes library on a Time Capsule so play music direct from Apple TV under 'Shared Music' but I have to have my notebook switched on to share the iTunes library.
So you could do it that way if you're not using XBMC. Put content on Time Capsule, add to iTunes library and stream it from your Apple TV. (although you already Sync content?)