it only shows up when the device is connected.
if you want to maintain a constant list for your Nano, the easiest way to do this would be to create a playlist(s) that only sync with your Nano and maintain that/them (if multiple ones, create a folder specificly for your Nano).
when your Nano is connected you can edit the lists that sync with it automatically too.
Unfortunately, for some reason only Apple knows, this option is only available for the Shuffle. It is not available for the Nano. (I have both a Shuffle and Nano.😛 )
I have a folder in my source list that I keep for my nano. Within it, I have created many playlists and smart playlists that i can modify when my nano is not plugged in. Then I only have these playlists synched to my Nano. It works really well for me and is quite easy to do.