hdiutil convert /tmp/HighSierra.dmg -format UDTO -o ~/Desktop/macOS_High_Sierra.iso
If you've downloaded the macOS High Sierra installer from the Mac App Store, you can create a bootable ISO file using the following steps: macos high sierra iso
This step takes 10–20 minutes.
You're looking for a macOS High Sierra ISO file. Here's what you need to know: hdiutil convert /tmp/HighSierra