You don't add "files" to Kodi. You add a "source" which defines the base path and type of file (TV or Movies) to be scanned for,
I slightly misspoke (miswrote?"); I said "files" because I navigated there through the "Add Files" screen, where I then attempted to add the source. Unfortunately, the sources given did not directly show USB mounted file systems, as it has on other platforms. I therefore had to start at the root directory and figure out where they were mounted by LibreElec since that information wasn't readily available -- as I noted, it's under /var/media.