I'll echo what many people have said. It depends on your game world. As far as I know, there isn't 'much' concrete evidence in regards to souls in 3.5.
Nonetheless, as far as I'm concerned, and I think for my world (though I'm still fine-tuning things), a soul is the embodiment of who you are. It is your mind, your sentience, etc.
Most living creatures that have achieved certain levels of sentience have a soul. The basic humanoid races. Animals. Hell, even plants have souls. For many Outsiders, their body IS their soul; it is one and the same. Ghosts are a manifestation of the soul. Certain undead (Vampires, Lich's) retain their soul, though in the latter case, they hide it in a safe place.
So for my world, to not have a soul at all, is to not be sentient, to not be. Unless something strange was at work, where your soul is connected to your body, but from the outside, then you could not live without a soul. You are the soul.
Again, this is the opinion of Alaris, mostly for his world. I'm not saying you should necessarily use it, especially since it does not work for your idea, but I figured I would give you my perspective.
Ooh, an interesting question. And one I think I want to answer. At least, in theory.
In my opinion, it essentially would go like this (for my world anyway):
- Dead Human is raised as a Zombie!
- Awaken [Undead?] is cast on him, giving him mental scores!
- Either a soul is bestowed upon him (by a Deity, or something), one is reincarnated into him (not standard rules), or a new soul is born.
- The now sentient Zombie is killed!
- The new soul, being the last resident of that body, is called back into it, if he so chooses to come.
Naturally, if some wizard started to abuse this to "create souls," the Deities may step in, as 'the soul' is not a domain for mortals to be toying with.
At least, that's my interpretation. ^_^