It seems that only the folder /photo is used by Photo Station.
The only workaround i have found (yes, good news, there is at least one !) is to create a symlink in the /photo folder pointing to the external folder to add.
Log in with SSH or Telnet to your Disk Station, cd to the /volume1/photo directory, and then create the link with the ln command, for example :
photos> ln -s /volume1/Archive/MyPhotos MyPhotos