Missing icon - How to add icons not available in the Xcode appicon template?

You could try manually updating the Contents.json file.

Here's what mine looks like with all icons, you can use it as a base to add the missing sections in yours:

{
  "images" : [
    {
      "size" : "24x24",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "notificationCenter",
      "subtype" : "38mm"
    },
    {
      "size" : "27.5x27.5",
      "idiom" : "watch",
      "filename" : "Icon-27.png",
      "scale" : "2x",
      "role" : "notificationCenter",
      "subtype" : "42mm"
    },
    {
      "size" : "29x29",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "role" : "companionSettings",
      "scale" : "2x"
    },
    {
      "size" : "29x29",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "role" : "companionSettings",
      "scale" : "3x"
    },
    {
      "size" : "40x40",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "appLauncher",
      "subtype" : "38mm"
    },
    {
      "size" : "44x44",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "appLauncher",
      "subtype" : "40mm"
    },
    {
      "size" : "50x50",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "appLauncher",
      "subtype" : "44mm"
    },
    {
      "size" : "86x86",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "quickLook",
      "subtype" : "38mm"
    },
    {
      "size" : "98x98",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "quickLook",
      "subtype" : "42mm"
    },
    {
      "size" : "108x108",
      "idiom" : "watch",
      "filename" : "[email protected]",
      "scale" : "2x",
      "role" : "quickLook",
      "subtype" : "44mm"
    },
    {
      "size" : "1024x1024",
      "idiom" : "watch-marketing",
      "filename" : "Icon-1024.png",
      "scale" : "1x"
    }
  ],
  "info" : {
    "version" : 1,
    "author" : "xcode"
  }
}