The normal way
First make sure that you have The Sims 2 University installed, after a sim dies if you want to bring them back you need to get your surviving sim to the top of the paranormal career track wich will unlock the phone to call the Grim Reaper, have you sim call and pay less than §5000, I suggest around §100 he will bring back the dead sim, your sim is now a zombie and will live forever.
The cheating way
This way is way faster since you don’t have to get your sim to the top of the career track, hit the ctrl, shift, c buttons in the cheat box type unlockcareerrewards buy the phone call the Grim Reaper and follow the steps I stated above. Your sim is now a zombie and will live forever
I hope I have helped you in your attempts to get a zombie in the sims two. When your sim is a zombie they will look grayish blue and walk with a limp but other than that they act just like a regular sim. The only way to get rid of them is to kill them off by fire or some other way like that.