How do I add my own skies?

In the Sky sliders, under the heading “Clouds” there is a row called “Replace With”, with an “Add…” button at the end. You can use this to add any image into the cloud replacement list. After doing so, you could save a preset containing those new clouds. If “separate clouds from atmosphere” is ON, it would just replace the clouds. If it is OFF, then the entire sky is replaced with your image.