How to list a published reply to editor in publication list
A conference paper, journal article, letter, book chapter, ..., they're all just publications, they can each appear as independent entries on your publication list. A blurb can describe relations between works, e.g., I pioneered X [1], with extensions to Y [2] and Z [3], where X is your original article, Y is your letter, and Z is, I don't know your work, something else you did in that area.