[Morthoron]

Originally Posted by Gothmog, LoB

That is plausible to an extent but we have to keep in mind that the people ensnared by the Nine were (quoting from memory) 'the kings, warriors, and sorcerers of old'. There was no Númenórean king among them, of course, so this category is off the table, and we also don't know whether there were any sorcerers in Númenor (that is not a positive profession in Tolkien's work, so one would look for those also among the men of Middle-earth).

That leaves the warriors. However, considering the life expectancy of the Númenóreans and the fact that Sauron would have been interested in learning more about Númenor it seems unlikely that none of the future Nazgûl didn't at least visit Númenor prior to their transformation.

Actually, in a footnote to Letter 156 (4 November, 1954), Tolkien wrote:

* There were evil Númenóreans: Sauronians, but they do not come into this story, except remotely; as the wicked Kings who had become the Nazgûl or Ringwriaths.

The emphasis is mine. Also, in another missive, Letter 131 (undated, likely 1951), Tolkien delineates the area of suzerainty of these evil Númenórean Kings, sub-kings and sorcerous lords:

The Númenóreans carry their evil also to Middle-earth and there become cruel and wicked lords of necromancy, slaying and tormenting men; and the old legends are overlaid with dark tales of horror. This does not happen, however, in the North-West; for thither, because of the Elves, only the Faithful who remain Elf-friends will come. The chief haven of the good Númenóreans is near the mouth of the great river Anduin.

Again, the emphasis is mine. So, we are talking far south of Lebennin, Pelargir and Harondor, even further than Umbar (not yet overrun by Corsairs). The King in Númenór would have limited sway over the self-professed Númenórean kings and lords of Far Harad; in fact, in HoME 12, The Peoples of ME in the "Tale of Years" for the dates 2000-3000 2nd Age, we find the mention of "The King's Folk establish lordships in Umbar and Harad and in many other places on the coasts of the Great Lands"; undoubtedly, one of those "lords of necromancy" would eventually become the Witch-King. Later mentions of Herumor and Fuinir being Númenórean Lords of the Haradrim and even the mention of the Mouth of Sauron being a Black Númenórean (which I find fascinating, as Tolkien referred to that race as "dwindled" centuries before MoS could possibly have been born), articulate Tolkien's continued association of Evil or eventually Black Númenórean kingdoms to the far South.

So, yes, the Númenóreans who became Ringwraiths were Kings, sorcerors or lords of necromancy. They, above all other races in Middle-earth, would have the necromantic knowledge spurred in Númenór and present even in a distant descendant like the Mouth of Sauron, who learned the Black Arts directly from Sauron himself. Even the "Good" Númenóreans created items "wound about with spells for the bane of Mordor", as the Blades of Westernesse could attest.