After failing a few hours to create a iso image, involving i. a. a tool to create a iso image and terminal/command-line ways to create a iso out of the downloaded ".app" installation file of the OS El Capitan, I've created a bootable USB-Stick (MacBook Pro - El Cpt.).
It appears that Windows does not recognize the file-system on the used USB-Stick, so I used HFSExplorer to extract the files.
What can I do with these files to be able to install the OS on VirtualBox ?
Or... is there any easier way to install the OS on VirtualBox ?