Edit the entitygroups and add the zombies to those groups.
Guppy is correct. For horde nights, you want to target the "feralHordeStageGS(N)" entity groups (where "(N)" is a game stage number).
There are quite a lot of them, so adding them to each game stage group is a PITA. One thing I have done, that works OK, is to search for specific vanilla zombies within these entity groups, then append an NPC after it.
Example:
<append xpath="/entitygroups/entitygroup[starts-with(@name, 'feralHordeStage')]/entity[@name='zombieArlene']">
<entity name="npcWhispererFemale02EmptyHand" prob="0.3" />
<entity name="npcWhispererFemale02Club" prob="0.3" />
<entity name="npcWhispererFemale02Knife" prob="0.3" />
<entity name="npcWhispererFemale02Bow" prob="0.3" />
</append>
This will append one of the female whisperer NPCs into all of those groups, after the vanilla Arlene zombie. If the entity group doesn't have Arlene, the NPC won't be added. By choosing which vanilla zombie to associate with the NPC, you can sort of game stage the NPCs - for example, a very powerful NPC could be associated with demolishers, so only spawn in later game stages.
(Note: That's the NPCPackHuman version of the female whisperer. I'm not positive what WotW uses, but if it's the CPHuman version, it starts with "human" not "npc".)