the post is a bit old maybe. But I found it trying to find a solution for the DDS importing issue on UE Mac Editor, and I was able to find a solution by myself and wanted to share it in case someone else reach this post.
I generated a DDS cube map image with the SpaceScape software, but when trying to import it into UE, it was always returning the message “Failed to import”, and since the NVIDIA Tools are only available for Windows, I started looking for an alternative without too much luck, until I remembered that I have installed a copy of GIMP and decided to give it a try, and oila! I was able to open the DDS file, then export it again, I selected RGBA8 as the Format and deselected the option “Generate mipmaps”, and then imported it into UE again and it worked!
Hopes this helps to someone else.