It seems the answer is no. It's not really people that go to the lifestream but their memories and their lifeforce. Their memories are scattered and cannot be put into a lifeless body. This is only assuming they are using the lifestream idea from Japanese philosophy. If they aren't then there is no answer.
^that answer is corrrect in most cases however, Sephiroth comes back at least 3 times throught the compilation and even though "technically" he isnt reborn he still does come back even if its only an "after-image" per se
^Well, in AC, when he came back, he overtook Kadaj's body. So, this shows that with a suitible body, it's possible to ressurect people from the lifestream.
Possibly. Certain people apparently can rresurrect themselvs. In Case of Lifestream White, Aerith says she can too, but chooses not to return.