Kleene plus in Thompson's construction
It's technically an "extension" in the sense that Ken Thompson's original paper (ACM DL subscription required) doesn't mention it. This is presumably because his regular expression compiler didn't support "+" syntax in 1968.

However, pretty much every textbook presentation of Thompson's algorithm (e.g. see Bruce Watson's 1995 review paper on FA construction) mentions it. It's likely that no one person "extended" the method in Thompson's paper, or the "extension" was done by Thompson when he decided to support "+".