In the Sky sliders, under the heading Clouds there is a tool called Replace With where you will find a drop-down menu containing the currently available skies.
Click the Add… button at the end 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.