How do you do that?

You don't, as far as I know. At least not functioning ones.

You can, however, have townified skins for each of the supernaturals and then you could say that any townie that generates with one of those skins is that particular supernatural. Then when you see them or know which ones generated that way, you could snag them or summon them to a lot, temporarily add them to the household -- with the Sim Blender or something similar -- then "turn" them with something like the Sim Transformer or any other similar modded object, then retownify them (again with the SimBlender or similar) and send them on their way. That's the only way I know of to have townie supernaturals. Aside from the grand vamps and high witches, of course.

Thank you.

Or, if you make your own townies in Bodyshop or Create-A-Sim like some people do, you can determine with a dice roll or some randomizer whether a townie is a kind of supernatural. I use the Sim Randomizer. After you make them in CAS, you can turn whoever needs to be turned, them make them townies with for example the SimBlender. For existing townies you can also use some program like Hook's Randomizer to determine supernatural traits (like, make it a 1 in 50 change or something). Or Random.org. This way you don't need to have special skins.
Anyway, what iCad said was right, the game itself won't spawn townies as supernaturals.

But...would it work to have random Aliens, as in..real ones?
When I created my own Townie pool, I forgot that I would have liked Alien Townies as well...

Aren't aliens just a custom skin and eyes?

No, you have to make them yourself. In fact, I just did that not so long ago because I missed them in my game as well. It's a pity you can't have alien bartenders or something.

Do you mean townies or NPC's? Townies are the sims that walk by your lot, NPC's have specific jobs or roles. You should be able to grab passing townies, make selectable, use the creature painting on then unselect them.

Yup, that and a face sculpt to make them look like the classic "Grey" alien.

Ok, right..so I could just give some sims the unlocked skin and eyes.

You can with a mod- Theo made one to unlock alien eyes and skin in CAS and for townies.
https://web.archive.org/web/2012021...tuff/index.html

Links are all dead so you can get them here:
http://simfil.es/216872

Can't make your own PT mod without Theo!...I just had to say it.

That's true Rose! And I have lots of randomly generated alien townies (including bartenders) thanks to Theo.

If you want them in CAS you don't need a mod, just the testing cheats and a capital N.

*I'll go make plenty...*

Oops, I was reacting to Rosebin, not ihatemandatoryregister! Yeah, aliens are just the skin and eyes (the game doesn't care what the face looks like). And thank you simsample! I guess I should have specified what I mean: you can't get an alien bartender without extracting the sim, unlocking the alien eyes and skin, editing them in bodyshop, downloading and installing Theo's improved surgery SimPE plugin and perform the surgery in SimPE. Luckily some of those things you only need to do once. I knew how to unlock the skin and eyes in CAS, had no idea someone unlocked them.
Oh, but does the unlocking also mean random townies and NPCs show up with the alien traits?

I was just wondering the same thing. I only had Theo's unlocked skins and eyes to create my multi-PT mod. After it was done, I got them out of my folder. So I would not know.

Yes, townies and NPCs will generate with the Maxis alien skintone and alien eyes using Theo's mods, although not necessarily at the same time. I have randomly spawned alien bartenders, townies, social townies, hobby mavens etc. To be clear, the alien eyes and alien skintone are not dependent upon one another- I have random sims with alien eyes and S1-4 skintones, as well as alien skintones with blue/ brown/ green/ grey eyes. So I can have 'aliens' without using Alt Sim Surgery.

I thought so, thanks for the clarification.