There is a cheat for it:
CTRL+SHIFT+C opens the cheat box
type on: Boolprop testingcheatsenabled true (type it EXACTLY like i did, with the spaces and all)
Shift+click your sim
Click change fit
there will be 2 options: fat or thin/fit
and that's it ^^
Chat with our AI personalities
You can not make a Sim anorexic.
You can't make a Sim beg for money.
No, elder Sims can not get pregnant.
My guess is that you can't make your sim to a vampire on the sims 3.
Well there is no cheat all you have to do is make your Sims do their homework. If you have Sims 2 Seasons you have your sim drink apple juice before they do their homework. It will help them do it faster.