Create a biome that generates in the nether?

This is not possible as Nether does not have biome manager and by core only supports one biome.

(how did nether-ex do it?)

Most likely with nether world gen modifications but such modifications are most likely not compatible with other mods. Nether uses single biome world gen provider by default. This would be out of scope of MCreator for now.

Late reply, but you can do it by creating a nether like dimension, and put your biome in it, then with procedures, you can check what dimension the player is in, and if they're in nether, then just simply change the players' dimension to your nether

i would think so

I know I'm very late but here is a screenshot that I took to show you how to do it

file:///C:/Users/Sponge%20Bob/Pictures/Captures/customstuff%20[C__Users_Sponge%20Bob_MCreatorWorkspaces_customstuff]%20-%20Portalenter%20(Procedure)%20-%20MCreator%202020.3%206_9_2020%201_38_18%20PM.png

your welcome.

Black_widowG You should know it as every other person in the world: If you send a pic link that starts with "file:///" then that will only be visible only on the computer that contains that picture in that directory.

oh oops i didn't realize that 

https://imgur.com/wWoOhXp

When I do this my custom dimension doesn't show up in the dimension field...

Never mind, I was being dumb. I made the biome but not the dimension...

Btw by making custom dimensions you are making your mod incompatible with every single other nether mod.

Also when MCreator updates to 1.16 would we be able to do it then since Minecraft has multiple biomes in the nether?

Also when MCreator updates to 1.16 would we be able to do it then since Minecraft has multiple biomes in the nether?

Yes of course!

1.16 has nether biome support. Thats why I can't wait for 1.16 mcreator.