To add bosses, go to difficulty_mob_health > functions > easy
Copy and paste an existing mcfunction.
Open your new function using vscode or your preferred code editing software.
Rename the function to the name of your entity. 
Change both of the HP values to your desired HP.
Save.
Go to difficulty_mob_health > functions
Open easy.mcfunction.
Copy and paste any of the existing code, should look like this:
execute as @e[tag=!health_modified,type=wither] run function difficulty_mob_health:easy/wither
Replace the firtst wither with the ID of the entity you want to modify (modded entities require the modname in front of the entity ID, the stonegolem from adventurez serves as the modded entity example)
Replace the second wither witht he name of the mcfunction you made in difficulty_mob_health > functions > easy.
Save.
Repeat steps for normal and hard difficulty.