Since version 4.1 of the Seafile client it is possible to synchronize single folders of a library. The name for this is Selective Sync.
To set up Selective Sync for a folder, double-click a non-synchronized library in the Seafile Client.
In the following window, navigate to the folder that you want to synchronize and right-click it. In the context menu, select Synchronize this folder.
The folder to be synchronized is then displayed separately in the Seafile client at the library level.