In the existing movies, there is nothing mentioning Predators adopting humans, however it has happened at least once in the comic book series, which predates the movies:
Machiko Noguchi (野口真千子), was a Chigusa Corporation employee and the administrator in charge of the Prosperity Wells colony on Ryushi. Unfortunately, this planet has been selected as a hunting ground for a young Yautja (the proper name for the Predator species): Predators plant a few xenomorph (Alien) eggs on a planet and send young warriors to prove themselves in the hunt. Unfortunately for both humans and Yautja, a Queen hatches from one of the eggs and quickly both the hunting party as well as human colonists are facing a huge swarm of Aliens.
Machiko oversees the evacuation and then helps one of the predators to kill the aliens - together they manage to kill a queen, however, the predator, now mortally wounded marks her as a warrior:
When the Predator ship arrives, Machiko is invited to join in and given honours of a hunter. However, with time she has to fight more and more prejudice and while she manages to keep proving her skills, she is demoted to being just a servant. When she finds out that on the current hunt Yautja attacked a human colony, she betrays Predators by releasing imprisoned on the ship Alien Queen and helps to kill the hunters.
More info at AVP wiki (Link already provided by @Valorum)