There are several ways to add new zombies from one mod to another. In this case with Snufkin zombies...
Option 1 - appending the new zombies to existing zombies in Snufkin mod across all spawner groups. Go into the entitygroups.xml file of Snufkins mod and use the following xml template:
<append xpath="/entitygroups/entitygroup/entity[@name='EXISTING_VANILLA_OR_SNUFKIN_ZOMBIE']">
<entity name="ENZOMBIE_NAME" prob="NUMBER"/>
<entity name="ENZOMBIE_NAME" prob="NUMBER"/>
... and more rows if desired ...
</append>
For example this can be:
<append xpath="/entitygroups/entitygroup/entity[@name='zombieBoe']">
<entity name="enDesertMale1" prob="0.5"/>
<entity name="enForestFemale1" prob="1.0"/>
</append>
<append xpath="/entitygroups/entitygroup/entity[@name='ZombieBomber']">
<entity name="enWastelandMale3Feral" prob="0.40"/>
<entity name="enFarmerFemale1Radiated" prob="0.25"/>
</append>
This tells the game to go through all spawning groups within entitygroups.xml and wherever it finds the vanilla zombie "zombieBoe", it will add the enZombies "enDesertMale1" and "enForestFemale1" after it, and using those spawn probability values. Likewise, whenever it finds the snufkin zombie called "ZombieBomber", it will add the enZombies "enWastelandMale3Feral" and "enFarmerFemale1Radiated" for a chance to spawn as well. this method applies to all zombie groups in the game, so it will search through all zombies groups like for wilderness spawns, screamer hordes, bloodmoon hordes, wandering hordes, and not just sleeper POIs.
Option 2 - appending the new zombies to Snufkin mod for specific zombie groups. Go into the entitygroups.xml file of snufkins mod and use the following xml template:
<append xpath="/entitygroups/entitygroup[@name='EXISTING_ZOMBIE_GROUP']">
<entity name="ENZOMBIE_NAME" prob="1"/>
<entity name="ENZOMBIE_NAME" prob="1"/>
... add more rows if desired ...
</append>
for example this can be:
<append xpath="/entitygroups/entitygroup[@name='ZombieBikerBarGroupGS1']">
<entity name="enBiker1" prob="1"/>
<entity name="enStripper2" prob="0.8"/>
</append>
<append xpath="/entitygroups/entitygroup[@name='ZombiesWastelandNight']">
<entity name="enWastelandFemaleCrazy" prob="0.9"/>
<entity name="enSwatMale2" prob="0.5"/>
</append>
ZombieBikerBarGroupGS1 is the zombie group that gets used for strip clubs and trailer park POIs. The above code will add enZombies "enBiker1" and "enStripper2" to also spawn as part of this group. Likewise, ZombiesWastelandNight is the zombie group responsible for spawning wilderness zombies in the wasteland biome at night. That XML example will ensure enZombies "enWastelandFemaleCrazy" and "enSwatMale2" have a chance to spawn there. FYI - the zombie group called "sleeperHordeStageGS1" is responsible for many of the sleeper zombies inside POIs.
As you can see, Option 2 gives you much more control to spawn zombies into specific areas of the map, but the drawback is it's a tedious process because there are many zombie groups inside entitygroups.xml. For example, the zombie group above called "ZombieBikerBarGroupGS1" also has ZombieBikerBarGroupGS50, ZombieBikerBarGroupGS100, ZombieBikerBarGroupGS200, ZombieBikerBarGroupGS400... where the numbers 50, 100, 200, 400, etc represent the players gamestage for which that spawn group will trigger. To view all the possible zombie groups out there, go into the entitygroups.xml file that's inside the vanilla configuration -- inside the /Data/Config folder.
Another option instead of the above, is to start with my enZombies mod as the base, and then add the Snufkin zombies to that. I've already done the tedious work of identifying all the possible zombie groups and packaged it into the Snufkin Zombies Add-on. All you do is download the enZombies mod and the Snufkin Zombies Add-on, then edit the entitygroups.xml file from within the Snufkin Zombies Add-on, and you are on your way. If you have more questions about this method, feel free to post up on my enZombies thread as I wouldn't want to muddy up this thread with enZombie mod troubleshooting, since it belongs to Arramus' Snufkin mod.