In most supernatural lore, vampires and werewolves are capable of passing their curse on to others. Given that I've made this a class-based system, that could get dodgy. Here's what I'm thinking so far:
Vampires can inflict their cursed condition onto others by draining them dry. (That is, to 0 Constitution)
Such an individual will rise as a vampire in 20 + 1d6 hours.
The fledgling has -2 to all saving throws or other forms of resistance to the sire's control. In the case of unwilling NPCs, the new sire can make a reaction roll at +2 to determine if the fledgling is loyal or rebellious.
When turning into a vampire, the individual's class will change.
Ordinary mortals immediately become 1st level vampires and advance normally thereafter. At least two of their powers will be the same as the siring vampire, though Black Magic takes 1d6 months before the fledgling can use it.
Heroic mortals become a vampire with an experience level equal to their Hit Dice -1. (Minimum 1) A 5 HD templar who becomes a vampire is a 4th level vampire, for instance.
Note that fringers are a special case. A fringer who becomes a vampire retains their current fringer abilities and gains the abilities of a 1st level vampire. In any session where they use their fringer abilities, including BHB or saving throws, they gain only half experience points. Once their vampire level exceeds their fringer level, they may use all abilities freely.
If a vampire tries to turn a supernatural creature, reference the rules below:
Werewolves: A werewolf cannot become a vampire. They simply die if reduced to 0 Con, although they have the normal chance to revive. (At 1 hp and 1 Con.)
Witches: A witch switches to the vampire class, at one level lower than their witch level. (Minimum 1) They must take Black Magic as one of their starting powers. They lose all witch abilities, and white magic spells no longer function for them. If they have a familiar, it abandons them. All Talismans the witch is maintaining lose power within 1d6 days. The witch advances as a vampire thereafter.
Cambions: A cambion loses an experience level and switches to the vampire class, advancing normally thereafter. Their cambion abilities are forever gone. Any thralls are immediately freed and lose any power the cambion has bestowed on them.
Weirdlings: A weirdling loses an experience level and switches to the vampire class, advancing normally thereafter. Their weirdling abilities are forever gone. Any contracts they have active are immediately canceled with no adverse effect to either party.
Revenants: Being undead already, a revenant cannot become a vampire.
Note that if a supernatural with fringer vassals is turned into a vampire, the fringer immediately has the option to break the bond, or they can simply switch to the vampire vassal benefit.
Werewolves can pass on their curse to another. If an individual is reduced to 25% hp or less by the werewolf's claws or bite, they must pass a Physical Saving Throw or they will transform into a werewolf at the next full moon. Note that individuals who are killed by a werewolf's claws and bite also have a flat 1% chance of reviving with 1 hp, doomed to become a werewolf at the next full moon.
If a werewolf turns an ordinary mortal, they simply become a 1st level werewolf, advancing normally thereafter.
If a werewolf turns a heroic mortal, that mortal becomes a werewolf with an experience level equal to one less than their hit die. An 8 hit die monster hunter becomes a 7th level werewolf, for instance.
Fringers are a special case. A fringer immediately switches to a 1st level werewolf, but retains all fringer abilities. The fringer advances as a werewolf, but loses 50% experience for any session where they use any werewolf abilities. Once their werewolf level has surpassed their fringer level, they can use all abilities freely with no penalty.
When a werewolf attempts to turn a supernatural creature, consult the following rules:
Vampires: Vampires are undead and cannot become werewolves.
Witches: Witches lose an experience level and switch to the werewolf class, in which they advance normally thereafter. Any familiar abandons them. Talismans lose their power in 1d6 days. The werewolf immediately loses all other witch abilities, including the ability to prepare and cast spells.
Cambions: Cambions lose a level and switch to the werewolf class, advancing normally but losing all cambion abilities. All thralls are immediately freed from service and lose any powers invested in them.
Weirdlings: Weirdlings lose a level and switch to the werewolf class, advancing normally but losing all weirdling abilities. Any active contracts are immediately canceled with no ill effects to either party.
Revenants: Being undead, revenants cannot become werewolves.
Note that if a supernatural with fringer vassals is turned into a werewolf, the vassals are allowed to immediately break the bond or switch their vassal benefits to those of a werewolf's vassal.