I've been looking into this for months. There's no "easy" way to edit the existing shaders, and there's no way to create entirely new shaders at all. What I can say though is that I don't think the ".hpss" file is what you should be looking at. The shader files themselves are in "core/shaders/hpsl", and are all have the ".hpsl" extension. I think "shadersource.hpss" and "shadercache.xml" are intermediary files that the game generates when it detects the shaders need to be recompiled, and if so, you shouldn't need to mess with it.
The shader language they use is a custom one that is based on GLSL, but
this blog post is the only place I've been able to find them talking about it at all. It does a good job at breaking the language down for people who want to write their own shaders, but doesn't provide any relevant information for how to actually utilize them in the game. All the existing shaders are used via helper methods in the script, but where those methods and the shaders interact seems to be buried in the engine itself.
If you come across any new breakthroughs, be sure to come back and let us know. Until then, good luck.