I am not running a NAS drive but I have bought an extra internal drive to do the same thing.
Here's what I did.
1. Copy the entire ITunes Folder to the new drive.
2. Set the perferences to in the Advanced tab to the new location.
3. Close program & restart.
Now I did this a number of years ago so I am not sure if this has been fixed or not. However, when I did it... any season pass you bought, will automatically continue to be downloaded to the music folder that Itunes first designated. Second, all Apps go the music folder too. So, after a few months of everything seeming to work ok.. I deleted the Itunes folder from my OS drive.. a lot of stuff was deleted too. From all the genius and tech support I spoke to, they thought that just changing the preferences would have work.
However, what I also needed to do was to set an symbolic Link of the extra hard drive and Itunes believes it is storing the stuff on the OS drive. Makes it easier to backup too. I bought a little script that makes alias a lot easier. But, I can't remember the name off hand. But is comes up as control click for me.
Also, in order for Itunes to see the NAS, you have to have it loaded in first. Again, I think you can start it up when you first turn on your computer in system preferences.
I hope I've help.. I know not completely but I hope I've pointed you in the right direction.