Adding a simple variant to a piece of clothing

I have a file (which is a vest) with 10 variants. The file name is:

And the variants are called:
And so on.

I changed one of the vests, and that works, but I wan’t to add a few vests, how do I make this work? I’ve simply copied one of the files, renamed it to the firs letter that was not in use, but I cannot select it in my server.
So the file I’ve added is called:

I also made sure to rename the texture file in openIV to the corresponding letter: task_diff_000_k_uni.png

I’ve read a lot of tutorials, but most of them are about streaming whole packs and seem complicated, I just want to add a few variants to existing clothing. Any idea on how I can manage this?