Use a tool like (free) or dmg2img (command line).
python macrecovery.py -b Mac-7BA5B2D9E42DDD94 -m 00000000000000000 download descargar mac os high sierra iso desde windows
Once you have the raw files, the real work begins. You can't just rename a file extension and hope it works. To create a functional ISO from a Windows environment, you typically need specific tools: Use a tool like (free) or dmg2img (command line)