You might post on Contributors Help and see what the experts suggest. I see he's not credited, so you could add that first. However, I assume it should be listed as "uncredited" and won't show up on the list. You could probably use [brackets] and explain there. It would mean using them inappropriately, but i don't see a clear cut way to list it. If that was the last line spoken, the brackets could be used to list it as [last line] which would be appropriate, then you could add the "-spoken by the real Sam Childers (uncredited)," within the quotes.