John L. Sorenson argues that the Book of Mormon contains strong implicit support for "others" being in the land when Lehi arrived and their being incorporated into the Nephites and Lamanites.
John L. Sorenson, "When Lehi's Party Arrived in the Land, Did They Find Others There?," Journal of Book of Mormon Studies 1, no. 1 (1992): 1-34
Abstract: A number of statements in the Book of Mormon text are examined, which indicate the presence in Lehi's "promised land" of peoples other than those descended from Lehi's party. Reasons are considered why the topic is not addressed more explicitly in the record. It is concluded that there is clear evidence for the presence of "others."