I've been meaning to do a write-up post discussing how this will be done, as currently we don't have any tools to give you folks to create your own mods.
The main issue is with the licenses that Epic Games has for UE4. Basically, we aren't allowed to distribute UE4 code, so we can't give you beautiful modders the source files you'd need to create your own Automation content.
The 'best' option would be to put together some tools ourselves that you can then put in to a default UE4 build and create assets with, but that's a lot of extra work, and a lot of Automation source assets we'd have to give out at the moment (I just did a test export of the assets you'd need to make a fixture, and it's like ~150Mb and ~200 files). I'm going to try and discuss this with Andrew and Cas when they get back and see what can be done with that.
Right now, I'm leaning towards asking very politely for the source .fbx files so I can bring them in myself, but there are So Many other things I need to do before I can have time to work on that.
tl;dr - We're still discussing the best ways to do this.