Will of the Nazgul/Creating More Nazgul

[Mithadan]

Appendix A states that Thrain's ring "was believed" by Durin's Folk to be the first forged of the Seven and "they say" that it was given to Durin III directly by the Elvensmiths. This section of the Appendix also states that Dwarven nature provided a resistance to some of the effects of the Seven and, as a result, they could not be reduced to wraiths or enslaved to Sauron's will and their lives were not extended. Nonetheless, the Seven did have evil effects upon the Dwarves because Sauron assisted in their forging.

To return to the topic, this suggests that it is possible the three remaining Dwarven Rings could have been used to create more wraiths. But the fact is that they were not so used, so either they were not appropriate for this use or Sauron chose not to use them this way.