A isn't a standard file transformation because these two formats serve completely different gaming architectures. While .z64 files are raw images of Nintendo 64 cartridges, .iso files are digital replicas of optical discs like CDs, DVDs, or Blu-rays.
An ISO file is a sector-by-sector copy of an optical disc (CD or DVD). In the context of N64 gaming, ISOs are usually created when users want to burn a collection of ROMs to a DVD to play on a modified GameCube or Wii using homebrew software. Why Convert Z64 to ISO? z64 to iso