At the very beginning of Narnia (in The Magician's Nephew), King Frank and Queen Helen (the cabby and his wife) were set to be the first King and Queen in Narnia. It also appears that some of their descendants were the first kings and queens in Archenland:
... you and your children and grandchildren shall be blessed, and some will be Kings of Narnia, and others will be Kings of Archenland which lies yonder over the Southern Mountains.
(The Magician's Nephew, chapter 11)
Further than this, and than hearing that the land of Telmar was populated by escapees from a shipwrecked group of pirates from Earth, we hear nothing specific about how humans came into Narnia and the other lands.
We are told incidentally, however, that there had been other ways of getting from Earth to the world of Narnia. The cave through which the pirates entered Telmar, according to Aslan, was
one of the magical places of that world, one of the chinks or chasms between that world and this. There were many chinks or chasms between worlds in old times ...
(Prince Caspian, chapter 15)
Thus, it's entirely possible that there were other humans who came into Narnia—especially considering that there need be no relationship between the (Earth) time when they existed and the time they came into the other world. But there is no canon discussing this that I'm aware of; and considering the audience for whom Lewis wrote the books, certainly it wasn't in his mind to discuss the difficulties and specifics of sexual reproduction in very small groups of people.