Once you have the skintones unlocked, you'd just extract them using Bodyshop and load them back in as custom skintones. Keep in mind that the skintones (except alien) don't have all the age groups, which means that sims wearing those skintones would not be able to have babies unless you create skintones for those age-groups yourself. If you have default replacements in at the time you extract the skintone, it will be the replacement and not original Maxis skintone that gets extracted. The vampire and witches are overlays and cannot be extracted this way - only werewolf, alien, zombie and plantsim.
The Bigfoot "skintone" isn't a skintone at all. Bigfoot's appearance is made up of a facemask, hair and full body outfit.
You can get access to all the skintones and the parts that make up Bigfoot's appearance by activating debug mode while in CAS.
Er, actually, one of the things treeag included in her download are all-ages CC versions of the creature skintones -- no debug mode needed and safe for breeding.
Unlockers for bigfoot hair and clothes, werewolf hair, plantsim hair, etc. -- to make them accessible in Body Shop and otherwise easier to get to without debug mode -- are around, usually on MTS2 or inSim. Just search for them one item at a time, since they are not grouped together in a single thread like treeag's skintones.
To complete my post, here's what I just did as an experiment.
1) Moved my My Documents\EA Games to my D: drive for safe keeping.
2) Ran Sim2 to create a band-new clean EA Games.
3) Exited and installed the supernatural families.
4) Reran Sims2 and moved the families into a brand new clean created neighborhood.
5) Exited and extracted the seven new people with the SimSurgery part of SimPE.
6) Copied the seven created files to D:
7) Deleted EA Games and restored my original one.
8) Moved those seven new files into SavedSims.
That gave me a werewolf, a bigfoot, a vampire woman, two zombies, an alien baby and a plantsim (I didn't want the baby's human father or the servo) but I can't edit them and the skins and stuff are not available. It works though. I have a Bigfoot to play with. Well, not a REAL bigfoot, vampire, zombie........but some look-alikes.