Afaik it's still not supported.
However, you could try the following:
In Tools / Options / Manage Files
enable Allow Songbird to manage files
and select your desired destination.
It will not move the files but copy over your whole library with the given naming convention.
So, before you try this, make sure to understand it's implications.
I have not used it from the start but doing it now. That way all my newly imported media automatically moves to that folder. It's not the watched folder feature though.