Can't mount writable images in Disk Utility
Let's assume you have a physical or virtual FAT32 or ExFAT formatted drive. You could mount the ISO file and then copy all the files to this drive. I would recommend using the Terminal application to do the copy. Using the Finder application may result in the creation of ._* files. Although these files can be removed by using dot_clean. 

To create a legacy bootable ISO file, use the command given below.

hdiutil makehybrid -o windows10.iso /Volumes/ESD-USB -no-emul-boot -boot-load-size 8 -eltorito-boot /Volumes/ESD-USB/boot/etfsboot.com


In this example, the name of the drive shown in the finder was ESD-USB. The -boot-load-size was set to 8 sectors, because the size of the etfsboot.com file was 4096 bytes and the sector size is 512 bytes. ( 8 * 512 = 4096 )