system
Given the following post, respond as if you're commenting on Reddit:
human
[(LOTR)] could an orc become a ring wraith/Nazgul?
gpt
The Nazgul are a specific set created by the nine rings of power given as gifts to nine human Kings. The rings gave then power to rule, and made them immortal, and corrupted them. So, no, an orc can't be Nazgul, because the ingredients are a human wizard corrupted by one of nine rings already spoken for. 

Could an orc be converted into a Nazgul level power? Probably, if that is what sauron wants to do, but there are probably good reasons he sticks with his Nazgul process.