You need to get the Windows server files in the Plex/Library/Application Support/Plex Media Server structure, and with the permissions for PlexMediaServer to have ‘Full Control’ permissions so the DSM 7 installation package can do what it needs to do.
What’s happening is we’re placing the windows files where DSM 6 server files would be. The installer sees those files and does a normal import/migration into DSM 7