as I answered in another forum (http ://answers.microsoft.com/en-us/windows/forum/windows_8-pictures/add-a-nas-photos-folder-to-win8-photo-library/4515a543-66ed-4191-9bec-fce1663c9f01)
1. Open Windows Media Player (I am using WMP 12)
2. Select "Organize" > "Manage Libraries" > select a library
3. You will find a dialog box "Change how this library gathers its contents". Clicking on "Add" you should be able to select and add a folder on your NAS.
Alternatively, since you are on a RT system, you can try this:
1.First you need the "UNC" path to your NAS share, could be something like: "\\NAS\Photo Collection"
2.Next create a folder on your hard disk with an "unusual" name; e.g. "C:\abcxyz". Integrate this folder as usual into your photos library.
3.Now go to the directory
C:\Users\\AppData\Roaming\Microsoft\Windows\Libraries\
Copy the file "Photos" on the desktop
4.Right mouse click on the "Photos" file on desktop, then "Open With ..." -> "Editor" Then you see a plain text file. Search for location of "C:\abcxyz" and replace it with the UNC path \\NAS\Photo Collection. Delete the following packages with the cryptic numbers between <serialized>...</serialized> Then the corresponding area should look like this:
<simpleLocation>
<url>\\NAS\Photo Collection</ url>
</simpleLocation></li>
5.Save the file "Photos". Copy the file "Photos" from the desktop back into the folderC:\Users\\AppData\Roaming\Microsoft\Windows\Libraries\
and override the existing "Photos" file.
6.Now you can delete the folder created in the meantime "C:\abcxyz" and the file "Photos" on the desktop.
...I hope it helps...
I am also interested to know if file indexing from NAS folder works in Win 10 TP... I'll try it sooner or later...