See Prince Caspian, chapter 15, "Aslan Makes a Door in the Air." The Telmarines actually are descended from pirates from our own earth, shipwrecked in the South Sea, who took native women for wives. Six of these couples, in the course of a fight in the group, fled to a cave in a mountain on the island; and in that cave passed through a magic chasm from our world to the world of Narnia. Since there are (or have been) portals between the worlds, it is possible that some others created elsewhere might have done so, as well.
In my understanding, from everything I've read in the Chronicles, Aslan is the form, in Narnia's world, of the One through whom all things in all worlds were created (cf. Colossians 1:16, John 1:3).
Regards.
P.S. After writing this, I checked Paul Ford's Companion to Narnia. I highly recommend it; it's filled with this sort of information - and much more, conveniently and very well presented .
