Adobe Photoshop can easily replace an empty sky with skies in Adobe’s library or your own. There are times when you’ve got a great photo but, because of timing or other circumstances, you miss a more interesting sky by minutes or hours or days.
I photographed a Red Tail Hawk against grey rainy skies. Because of the bright backlighting, I added +2 stops compensation. So the bright, flat sky, was completely washed out. To add a more interesting sky, I used Photoshop’s Sky Replacement (Menu: /Edit/Sky Replacement) to replace the background. I also applied a Filter/Gaussian Blur to soften the focus on the crisp clouds, adding bokeh.
Purist will scoff at using tools as this but there are times and circumstances when this is fine in my photography. As long as I’m honest about what edits I’ve done and they aren’t presented as untouched. I grew up with photographic film artists like Jerry Uelsmann and Man Ray. Their photographic manipulations were part of their vision, creative process and integral to their photographic process.

If you prefer to use your own sky photos, once the sky replacement dialog box opens, click the v next to the sky thumbnail then the + in the lower right. A file select dialog box will open, select the photo – it can be a raw file – then click Open. This will add the sky to the existing library. Skies can be manipulated, scaled, flips, rotated, and edited. Although, you may want to do all your edits before importing the image into the library.

Adobe’s instructions on using your own skies:

“Get the sky you love in three easy steps.” Adobe

Note – the following text if from Adobe’s sky replacement web page:

The Sky Replacement tool uses artificial intelligence to insert a new sky in place of the sky in the original image. Thanks to Sky Replacement algorithms, Photoshop can analyze and recognize the current sky and then plug in a new one. Upgrade your original sky in three steps:

1. Go to Select and choose Sky from the drop-down menu.

2. Choose Edit › Sky Replacement.

3. Use the sky drop-down to select a new sky from one of the preset sky images.

Adobe Photoshop offers a collection of skies, organized in three categories: Blue Skies, Spectacular, and Sunset. Blue Skies include daytime skies in solid blue or with clouds. Spectacular skies range from dramatic storm clouds and sunscapes to rainbows and a night sky. Sunset features yellow, orange, pink, and red sunset hues.

You can also import and use your own sky image to replace the sky. To do this, click the down caret and then the plus sign — you can import any JPG or PNG image.

“The best thing about Sky Replacement is that everything remains editable via the traditional Photoshop workflow,” says photoshop expert Jesús Ramirez. “This is not like a filter where once you apply it, you’re stuck with it. You actually have a lot of control, and all these controls eventually become a layer mask or adjustment layer.”

https://www.adobe.com/creativecloud/photography/discover/sky-replacement.html#:~:text=Get%20the%20sky,or%20adjustment%20layer.%E2%80%9D