Like their already-captured counterparts (v. 17), these Lamanite prisoners have the option of joining the Ammonites. Perhaps significantly, Mormon does not specify that Moroni offered the option but rather that the prisoners themselves desired it, suggesting that they had knowledge of the fate of the Ammonites (now in Melek). Again, they gave their parole not to take up arms against the Nephites and were apparently very industrious in providing food for the army. Both in increased food production and the removal of a need to guard prisoners were “the Nephites relieved from a great burden.”
A question Mormon never answers is the religious status of these prisoners. Their oath and support for the Nephite hegemony made them political Nephites, but did it change their religion? Perhaps Mormon preferred to imply that they were also religious converts, but there is no textual indication either way. Certainly the religious climate in the land of Zarahemla allowed for diversity of religion, so there is no reason to suppose that they were also converts. I suspect that the typical process would be that they retained much of their fathers’ religion for the first generation and that, in subsequent generations, the religious beliefs were more in conformity with the Ammonites among whom they lived.