The Collaborative International Dictionary of English v.0.48:

in-migration \in-migration\ n.
   migration into a place.

   Syn: immigration.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

in-migration
    n 1: migration into a place (especially migration to a country
         of which you are not a native in order to settle there)
         [syn: immigration, in-migration]