I've read the first two books thus far and enjoyed them. I think using heroes other than Conan is a good idea for avoiding the "it's not Conan if Howard didn't write it" stigma. Also, since Hyboria is so huge, there's plenty of room to explore. From a RPG perspective, though, I think they're a great example of showing how to avoid the "Conan's been there, done that - what's left for the PCs" syndrome that some players experience when they come to the game.
As to simulating Kern, he was raised Cimmerian so he has the cultural benefits of any Cimmerian. He's half Ymirish, which is a line of northmen that are supposed to be of Ymir's blood. The Ymirish aren't Frost Giants. They do, however, seem to have the potential for tapping into sorcery due to their closer bloodline to Ymir.
If I would create Ymirish characters, I would grant them some type of cold resistance as either a racial characteristic or a mandatory feat at first level. To reflect their ability to tap into the sorcerous power of Ymir's bloodline, I would simply allow them to treat Scholar (Sorceror) as a preferred class (but not at first level) and would not require a tutor to multiclass into Scholar. When the character wants to become a sorceror, he simply learns how to tap into his sorcerous lineage.
I'd create a PrC with prerequisites: BAB +5 and Ymirish template (= cold resistance + Eyes of the Cat + Animal Affintiy, but CHA -4 / mali to CHA-realted skills outside of Nordheim - you look like a freak)
The PrC would include several supernatural abilites described in the book.