Yeh it's kinda tough to make brand new costumes without the proper tools, or even with them tbh.
The costumes in Iris are very static, and by that I mean they are not just a few textures and models that get dynamically wrapped to all the different character models (human male and female, elf male and female, hybrid male and female), there is actually a model and texture for each costume and for each character type, so for costumes there are 6 models and 6 textures for every piece of costume/armor.
The NPC textures are designed specifically for the NPC models (shape, size etc) and can't be easily adapted for characters. We could take an existing costumes model and texture and try and apply the NPC texture to it but it's not an easy task to redesign these textures.
We do actually have one brand new costume that you haven't seen yet, well you might have seen a GM in it at some point. Not sure when I will release it, was kinda saving it for something I might do later on.