UPDATE #2: SimGuruNinja confirmed that, although we won’t be able to create Skeletons in CAS, our Sims will be able to become one.
Skeleton-ism (that's a term, right?) is an affliction your sims endure. You can't create them in CAS.
— Dave Miotke (@SimGuruNinja) February 14, 2018
If you can get them into CAS then hats and accessories can be put on them, but those additions will not appear when re-entering Live mode.
— Dave Miotke (@SimGuruNinja) February 14, 2018
UPDATE: I managed to uncover a deleted profile picture by SimGuruNick where a Skeleton is rocking a hat, glasses and the earbuds from The Sims 4 Fitness. This means that not only we’ll be able to create Skeletons in Create A Sim but also dress them up with Sim clothing!
SimGuruNick updated his profile picture to a picture of a dark skeleton from The Sims 4 Jungle Adventure Game Pack. Looking at the image it seems that it’s taken in Create A Sim, meaning that we’ll probably be able to create our own skeleton and play with it in Live Mode!