Yes, it is possible to create custom skyboxes and add them to the Skybox drop down menu in the 3D View. 

To create a skybox, you literally need a box of images (6 images total); 4 images around the sides, a top and a bottom. These images need to:

1. Have exactly the same number of pixels as each other.
2. Be saved in JPG or PNG format.
3. Be named in the format of: name_position.jpg. For example: alpine_zneg.jpg.
   1. The name is the name you want to appear in the Skybox drop down.
   2. The position is the position of that particular image within the skybox (see image below).
4. Be saved in the SkyBoxes directory (C:\Users\(username)\AppData\Roaming\Golden Software\Surfer 16\SkyBoxes).

 For the naming position, each image needs to be identified by where in the skybox it belongs:

For the best results, the images or photographs used need to be true 3D images. Otherwise, when Surfer assembles the sub-pieces to create the composite image, the joins between the photographs will be visible and will detract from the (ideally) seamless output.

There are a number of places online where you may be able to download prepared skyboxes. You can try the list below, or try a Google search.

• http://www.custommapmakers.org/skyboxes.php (they're in TGA format, so you’ll need to resave as JPG or PNG)
• https://www.assetstore.unity3d.com/en/#!/content/24923
• https://reije081.home.xs4all.nl/skyboxes/
• http://www.humus.name/index.php?page=Textures&start=0

Updated October 24, 2018