How to add an "?" Or "other" season episode

I want to add episode(s) of a show that are not in any season but also aren't in-between seasons either. On some shows (Using attack on Titan as an example: https://www.imdb.com/title/tt2560140/ ) you can view an "Other" or "?" With episodes that are not a part of any season whatsoever. How can I do this?

Thank you