I imagine the cases of Beren and the Nazgul were differnt, as Beren wes really dead and his fea left the body and lingered in Mandos waiting for Luthien.
But the Nazgul, I believe, did not have a chance to die, as their souls were posessed by the Nine rings and thus bound to Arda. Their bodies waned as they were the bodies of mortal men, but their fea stayed tied to them through their rings. That, I think, explains that the Nazgul do not have the abilities of "normal" ghosts: They need horses or other beasts for moving around, they couldn't penetrate walls or other solid objects... And they could be finally killed when what remained of their body was wounded by a charmed weapon (like the Witch-King's) or the rings which sustained their "life" (existance in this world) were destroyed.
So even if Sauron practiced necromancy - and he probably did - I don't think he applied it in case of the Nazgul. At least he didn't bring the Witch-King to life again. Or was he just too busy at the time?
