If these are videos in mp4/m4v format, you might be able to embed the most important pieces of info into the files themselves. Namely the title, a description, a poster, and maybe a genre, or an album/collection title as embedded meta tags.
Most of these will be imported into Plex, if the server cannot (or is forbidden to) match the file to an external metadata source.
You can use tagger software like mp3tag to do that.
(here’s an introduction into its use for video files in Plex: MP4 Metadata (Tags) for Personal Media Shows )
