Here the earliest text has the modal will. The 1874 RLDS edition replaced it with shall; the 1908 RLDS edition restored the original will to the RLDS text. Either reading is theoretically possible here, so we follow the earliest reading (“they will be the means of bringing many thousands of them ... to the knowledge of their Redeemer”). There is one other example in the text with similar phraseology, and it has will rather than shall: “for ye know not but what they will return and repent” (3 Nephi 18:32).
Summary: Maintain in Alma 37:10 the modal verb will, the reading of all the earliest sources (“and who knoweth but what they will be the means of bringing many thousands of them ... to the knowledge of their Redeemer”).