Yes, but the trigger for the transformation itself was when she was hit by L'ura's essence. Her training with Locus-Walker helped her control the Void powers she had, but it was being hit by L'ura who caused her to transform.You forget that Alleria has been under Locus-Walker's tutelage to master the void for quite some time before that. "Becoming a void elf" is not the same thing as eating a big hamburger.
And yet a Kirin Tor mage didn't do that, it was a void elf who did that. That doesn't take anything away from them.They did nothing a Kirin Tor mage could've done.
Also, they used their powers to send enemy troops into the Void, something that no mage can do.
Besides, I really don't see why it's a huge deal that void elves don't appear against N'Zoth, when orcs and blood elves didn't do jackshit against Kil'jaeden and in the invasion of Argus.
You literally saw how some void elves were created in their recruitment scenario, and furthermore they were also introduced prior to the expansion, as they are marketed as a Legion allied race. Their recruitment happens at the end of Legion, not at the beginning of BfA.Lightforged draenei at least were introduced in the expansion prior and got their development, and their recruitment scenario gives you some insight on the process of converting draenei into lightforged.
Nothing of the sort was ever done for void elves.