If the movies were purchased from a streaming service they most likely have Digital Rights Management (DRM) protection. Plex will not play such media.
Assuming they do not have DRM, you would need to make the movies available to the system running Plex Media Server.
You might be able to connect the external drive to the Plex Media Server and copy the media files to the Plex system (connect the drive to your PC, can you see the files?).
If the TV box has a way to share the files over the network, then the system running Plex Media Server might be able to access them.
