If you are new to using MO2 with 7DtD, read this other article first: How to use Mod Organizer 2 (MO2) for 7 Days to Die

How to add other Overhaul ModPacks to 7DtD using MO2

There are two methods:
 
1.     Add it to your current 7DtD MO2 instance
 
A.    You download the Overhaul ModPack (like Darkness Falls, Rebirth, or EFT Overhaul. 
 
B.    You add the zip file of that ModPack to the MO2-downloadsfolder.
 
C.   In the MO2 App, you drag the ModPack from the right-side of the screen in the Downloads tab over to the left-side of the
screen of the listed Mods. 
 
D.   Disable other non-compatible mods, enable the Overhaul ModPack.
 
E.     Run. 
 
Suggestion:
When switching Overhaul ModPacks, you should make a screenshot of your enabled ModList prior to disabling anything so you
can easily reenable the correct mods when you want to go back to your other game.
If you don’t use too many mods, it’s easy to remember which ones you use with which Overhaul ModPack. 

However, if you’re like me, and use tons of mods (in Smorgasbord), I would recommend the second method.
 
2.     Create a new instance of MO2
 
A.    Download an additional copy of MO2, install it in a separate location folder location different to your other MO2 installation(s). 
You could create a separate instance for each different Overhaul ModPack if you so choose.

Examples:
D:/MO2-Smorgasbord
D:/MO2-DarknessFalls
D:/MO2-Rebirth
D:/MO2-EFT_Overhaul

B.    Add the community-made 7DtD plugin to each unique MO2 installation.

C.   Download the ModPack for the Overhaul to the correct MO2 Instance Folder.

D.   You add the zip file of that Overhaul ModPack to that MO2 instances downloads folder.

E.     In the MO2 App, you drag the ModPack from the right-side of the screen in the Downloads tab over to the left-side of the
screen of the listed Mods. 

F.     Disable other non-compatible mods, enable the Overhaul ModPack.

G.   Run.  

I personally run two MO2 Instances for 7DtD. 
One for my Smorgasbord ModList, and a second Instance where I run other Overhaul ModPacks out of. 
This is because I don’t tend to add too many additional mods when I play other people’s Overhaul ModPacks, so it's easy to keep track of.  
So instead of trying to remember which mods I currently need enabled/disabled for my Smorgasbord ModList, I just use my second MO2 instance to play other Overhaul mods.