[jjlr]

I have a folder with a full video and some extras as separate files. Kodi links the large video file but not the extras.

How do I add the extras?

In Plex there was a fairly strict way to rename the files. Is there any easier method in Kodi?

[DarrenHill]

One way is the Extras add-on which you can get from the context items section in the official built-in repo.

Otherwise it depends a little on what the extras actually are, and whether you're talking about TV shows or movies. For TV shows, sometimes the extras are listed in the scraper database in their own right (usually as specials) and you can put them in as normal files with a suitable series/episode (S00Exx) set-up depending on how they're listed in the scraper database.

Generally the add-on is the simplest way to go, and you then put them all in a sub-folder from the one containing the movie file, and access them via the context menu (details are in the link above).