Hi 

There is no sync server but you can tell CRM to store any file in a different place to the default (which is within its own main folders). 

Go to Administration | Email and Documents | Documents & Report Configuration and edit the first to fields (Physical Root Directory and Default Document Template)

In short, documents will need to reside on the drive where the product is natively installed, typically the OS (C:\) Drive; i.e. UNC paths to another server does not appear to be supported.  

Never seen it anywhere that says it is not supported, it's rare but we have a few using UNC paths. 

I think this in reference to these fields:

And the help file is useless: