Apparently this feature available. First of all to do this you need to ensure Resilio Sync context menu is activated (https://help.getsync.com/hc/en-us/articles/206214625-No-Sync-icons-in-the-file-browser-no-Sync-related-items-in-the-context-menu-on-Mac-Windows). Once activated right click on one the folder within your Selective Sync folder and select "Sync to this device" on the context menu. Once activated all existing and future files will be automatically in synced.
What will be helpful though is if it can show Icon to which folder which is running on full sync.