To me, what the alien does up implantation is take specific traits from its host, and any "lesser" qualities are thrown to the side and replaced by WHATEVER the "pure" xenobiotic gene would be. Hence we have differneces in warrior, runner, and predalien (I suppose the Queen could be house nothing but pure genes, as we don't know for certain if the queen would be different from a dog, human, predator, or whatever).
ANYWAY, this makes the creature superior to its host genetically, enabling to it survive and destroy its host species. If this is the case, why would we try to splice human genes BACK into it? That is quite possibly the worst idea in the franchise. Whedon, I like firefly, but please never venture into sci fi again.