Use network volumes as Time Machine drives
From Tech-Wiki
If you already has some kind of personal Storage like NAS or some USB hard disk plugged into another computer in your home (or workplace), sure you can use this available space for Time Machine bacukps.
Or if you own some desktop computer with plenty free space and wish to backup your macbook through the network, just type this command at the terminal:
$ defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1