Well i think that we should have some sort of bookkepper like position that takes care of population, and with that, inmigrants, basically this guy would be the one that would count your population, maybe even be the one that counts the votes, he would be the one to inform about new children and so on, maybe in the future he would have even more important functions.
This guy would control inmigration in two ways, first do some sort of list of quantity of people and jobs you need, and then control inmigrants, i mean i dont think its impossible to expulse someone from a settlement, hell maybe even get him into jail, or something like that, also this should give you a quick way to give jobs to the inmigrants instead of having to search for them by yourself in the list.
I can think of two other ways to do this, either this task could be taken by the manager too, i mean its related to him after all
Or you could apoint a liaison yourself, hell you should even be able to send out a caravan yourself, why not?, but anyways, lets say you need some material, like an anvil and you need 2 carpentes, you inform this guy of that in a similar way to the normal liaison and he comes back with them, you could even pay some money to these inmigrants to make it easier for them to come.
I think the liaison and some sort of inmigration officer could both exist and be quite handy, i mean who dosnt want to get that soap maker you hate into jail, or execute him, or simply expulse him from the fort!