The actress will play a nursery maid called Beryl Ballard when the show returns for its second series.
Beryl is described as a "modern young woman" who is hiding a "tragic secret". When she ends up getting involved in a romance, she has to rethink her plans.
Haddock said: "I am so excited to be joining Upstairs Downstairs. It really is a dream come true. I loved the first series and when I found out I was going to be part of the second I couldn't have been happier."
She added: "My character Beryl Ballard is a really spirited modern young woman and all set to shake things up in the house. I can't wait to play her."
Haddock recently appeared in The Inbetweeners Movie and has also had television roles in shows like How Not To Live Your Life and Monday Monday.
Alex Kingston and Kenneth Cranham have already signed up for roles in the new series of Upstairs Downstairs, while Dame Eileen Atkins has decided to leave the show.