Luckily we support KiCad natively, so the steps are very very simple ;-) Right now we support the latest release of the KiCad 5.0 branch. If you use a nightly build, we will use this instead.
Since it is that simple you can also donate to CERN Foundation & Society – the sponsors of the KiCad project – when you order through us. They help keep this project alive. Of course, you can opt to remove that donation, but that kind of would make you a bad person, right?
If you want to have all the design rules etc. already set up in KiCad you might consider using one of our templates. Have a look at this Github repository.
We support KiCad file format natively. Just Drag'n'Drop or Upload your KiCad-Board file and you are good to go.