This method of secretly adding cards under a card or cards as they are gathered from the table was published in a one-page collection of ideas, L. W. Random Conceptions with Cards by William Larsen and T. Page Wright; see fifth paragraph. The sheet isn't dated, but ads for it, proclaiming it a new item, began to appear in June 1930 (e.g., The Sphinx, Vol. 29 No. 4, p. 190). In Page Wright's Notebook, published posthumously in 1933, there are several ideas credited to William Larsen that use expert thumb riffling. This tempts speculation that the Scoop Addition was his, but that is all it can be without sounder evidence.
Ross Bertram reinvented the sleight in the late 1950s or early 1960s. He used it in a Canadian television show produced by Don Hudson; see Genii, Vol. 66 No. 3, Mar. 2003, p. 34. René Lavand also conceived this idea independently, publishing it in The Mysteries of My Life, 1998, p. 28.
Stanley Collins devised a handling of this type of addition that took place entirely in the hands and off the table. He recorded this sometime from 1958 to 1963 in Gems of Personal Prestidigitation. This book remained unpublished until 2002, when it was included in Stanley Collins: Conjurer, Collector, and Iconoclast by Edwin A. Dawes. The addition is described in a short paragraph within “The Kappa Four Ace Trick”, p. 22 in the “Gems of Personal Prestidigitation” section of the Dawes book. (The addition was first noticed by Jerry Sadowitz.) The Collins add-on can be considered a transitional sleight between the Scoop Addition and secret additions in which the top cards of the deck are carried under a fan or spread of cards held in the opposite hand.