Yes, there is a term and it is known as "Reverse" Chuck Cunningham Syndrome"
A reverse Cunningham is essentially the opposite of Chuck Cunningham syndrome, in which a previously-unknown and un-alluded-to character is introduced under the context that they had always been there, and had significantly interacted with the other characters before. This causes even more continuity problems than regular Cunningham Syndrome, as although it's easy to overlook the sudden absence of a character, it's harder to ignore the sudden appearance of a character, especially one that we are expected to believe was part of the regular cast, all along. Perhaps because it creates even more credibility problems, producers are much less likely to suddenly introduce characters whose presence should have been at least alluded to or explained earlier. But it has happened on a few occasions.
A handful of examples is also given there at the end of the article.