The Kodi Plex add-on is a client with a sole purpose of playing media stored in a Plex server. If you want the media to be visible in the Plex client you need to add it to the media library in the Plex server. If you run a Plex server on the same HTPC host as Kodi (and the Plex client in Kodi) you can map media from local drives. If you run the Plex server somewhere else; you'll need to remotely mount the drives so that the remote Plex server host can see the media on them to add into the Plex server library. Kodi doesn't follow a rigid client/server model in the same sense; the GUI just provides access to media wherever you store it, and in your install that media is local which is easy.
It is not possible to change the behaviour of the Plex add-on without writing code to change the behaviour. As the current add-on is created by Plex staff the behaviour is intentional and I doubt changes to access local media would be welcome.