For ages I've wanted to add families that come with a simbot, or an alien, or a mummy, but CAS only allows for certain supernatural types to be created in CAS. Is there a mod that opens up CAS to all the occult types?

Maybe I'm missing something obvious here (wouldn't be the first time) but NRaas MasterController together with its MC Cheats module allows you to add an occult state to an existing sim. So I would think the way to do this if using NRaas mods would be to create your household first, put them into the game, add the occult states that cannot be reached otherwise, and return the individual sims to MC's version of CAS for final makovers and adjustments.

Aliens are different, those require a % of Alien DNA to be set, which can also be done by MC with the MC Cheats module once the sim is in-game. I believe anything over 20% is officially an Alien.