Actually in Linux if to try add existing folder - it is not possible. It opens the folder and then demands name which actually was the folder I typed but as it existed, it opened. When I type again same name then it creates folder with same name into existing folder which I did not want. I just wanted to add existing folder. Only way is to create folder during vault adding process and if such folder existed, rename it before adding vault.
Why not just allow to pick up existing folder and add it?
Noticed from introductory video where in macOS the given name folder was created inside Dropbox folder. So I need to log off from Dropbox, rename the ~/Dropbox folder to something else, create vault and during that vault creation the folder ~/Dropbox and then finalize the data moving from old Dropbox folder to vault. I need also clear Dropbox folder in web interface during I am logged off in computer client - otherwise after logging in computer client, Dropbox starts to sync files back from server to computer as it sees empty ~/Dropbox folder in computer. After adding encrypted files, Dropbox will sync them to its server.