...why?
why not it might cause their pop to sky rocket
In the current version, anyone who is a member of a civ will not reproduce outside of your fortress. Actually, wildlife populations don't replenish either. So there's nothing we can do to cause their population to increase at the moment. This will be fixed in DF2014 (should it be 2014).
In future versions of the Bloodkin race, giving them a reaction to instantly convert humans, elves and goblins kidnapped by Bloodkin childsnatchers upon sieging your fortress would mean more available Bloodkin per siege (as a significant number of children are kidnapped during worldgen - goblin populations sometimes go extinct, but their civs continue on as the descendants of 'culturally goblin' dwarves) although it still wouldn't replenish in-game. It would also be a tad glitchy since it would convert any goblins, elves or humans already on the map into Bloodkin.
This could actually be added now to give us that 'apocalyptic Bloodkin horde' moment we've all be waiting for when the 'Kin show up (which we can force using DFhack), although it could potentially royally screw us over when that human siege, goblin ambush and elven caravan all decide to show up at the same time we get invaded, suddenly spawning hundreds of Bloodkin. This also wouldn't spawn any more Bloodkin mages, unless it turned elves or something into thaumaturgists...
Not that I'm saying I can mod such a monstrosity, of course.