This really belongs in the "Discussion and Requests" forum... but anyway.
The sleeper volumes, used by prefabs, consist of three main components defined in the XML.
1. Gamestage groups. These are the groups that are actually displayed in the prefab editor UI. In gamestages.xml, they are defined with a <group> tag. Here is the gamestage group used in hospital POIs:
<group name="S_-_Group_Hospital" emptyChance="0" decoyChance="0" decoy1hpChance="0">
<spawner name="HospitalHorde" count="5,6"/>
</group>
2. Spawners. These are used by gamestage groups to choose which group of zombies will spawn, by gamestage. In gamestages.xml, they are defined with a <spawner> tag. Here is the spawner that is referenced by the hospital gamestage group:
<spawner name="HospitalHorde">
<gamestage stage="1"><spawn group="zombieHospitalGroupGS1" num="1" maxAlive="1" duration="1"/></gamestage>
<gamestage stage="50"><spawn group="zombieHospitalGroupGS50" num="1" maxAlive="1" duration="1"/></gamestage>
<gamestage stage="100"><spawn group="zombieHospitalGroupGS100" num="1" maxAlive="1" duration="1"/></gamestage>
<gamestage stage="200"><spawn group="zombieHospitalGroupGS200" num="1" maxAlive="1" duration="1"/></gamestage>
<gamestage stage="400"><spawn group="zombieHospitalGroupGS400" num="1" maxAlive="1" duration="1"/></gamestage>
<gamestage stage="800"><spawn group="zombieHospitalGroupGS800" num="1" maxAlive="1" duration="1"/></gamestage>
</spawner>
3. Entity groups. These provide the list of actual zombies that will spawn, along with the percentage to spawn each zombie. In entitygroups.xml they are defined with an <entitygroup> tag. Here is the entity group used to spawn zombies into hospital POIs if the player (or collectively players) are at gamestages 1 through 49:
<entitygroup name="zombieHospitalGroupGS1"><entity name="zombieLab" prob="1.7"/><entity name="zombieMaleHazmat" prob="1.7"/><entity name="zombieNurse" prob="2.5"/><entity name="zombieNurseFeral" prob="0"/>
<entity name="zombieBoe" prob="1"/><entity name="zombieBoeFeral" prob="0"/><entity name="zombieBoeRadiated" prob="0"/>
<entity name="zombieArlene" prob="1"/><entity name="zombieArleneFeral" prob="0"/><entity name="zombieArleneRadiated" prob="0"/>
<entity name="zombieYo" prob="1"/><entity name="zombieYoFeral" prob="0"/><entity name="zombieYoRadiated" prob="0"/>
<entity name="zombieMarlene" prob="1"/><entity name="zombieMarleneFeral" prob="0"/><entity name="zombieMarleneRadiated" prob="0"/>
<entity name="zombieSkateboarder" prob="1"/><entity name="zombieSkateboarderFeral" prob="0"/><entity name="zombieSkateboarderRadiated" prob="0"/>
The entity groups used by TFP are most likely generated programmatically which is why there are some with prob="0" (meaning they won't spawn at all).
So, to add custom zombies to these sleeper groups, you add them to the various entity groups used by the spawners. You should make sure you give them probabilities that match the probabilities of zombies that are already in those entity groups.
Hope that helps.