Humans can turn into supernaturals, but it is extremely difficult.
Vampire:
For a Human to turn into a vampire, he/she must get the blessing of each Lord and Lady of the four courts. If accepted, the human must make an offering to the leaders before the next full moon, whether it be their own blood or another's. Said human must then be locked in the Court's dungeon without food or water and only the cattle, if the human doesn't feed on the cattle by their third day, they are immediately executed and ripped to pieces by the court's shadow hounds. Should they succeed, he/she will be bathed in the sacred river and treated with care for one night.
After the night is through, the human must strike a deal with the witch coven to make a vampire's potion, which will prepare the blood for vampirism. (If said human doesn't use the potion, chances are they will not survive the transformation) And bring it back to the court.
Each member of the court has to provide a few droplets of pure blood, none of the hybrids or mixed-bloods can give any, and when all of the blood is collected, the human must down the entire thing and then use the vampire's potion for the next step. After drinking the potion, the lord of the court will bite the future vampire's neck, and mark them with a special sort of "tattoo" for them to wear as a symbol of the court. Should the human survive the traumatic transformation, he/she will be welcomed with open arms.
Werewolf:
A human must challenge the Alpha to a brawl, and if they (survive) win, they will be bathed in the river and placed within the guardian stones. The Alpha will offer his/her blood and turn into their crinos form, the human must bite the Alpha and drink their blood. The next mating moon will bring the human's wolf form and selected powers.