A very simple way of creating a GRUB menuentry for a bootable ISO file is to use grml-rescueboot.
The package does the following:
Creates a /boot/grml folder.
Automatically adds menuentries to the GRUB menu for any ISO files located in the /boot/grml folder. This is accomplished whenever the update-grub command is executed.
sudo apt-get install grml-rescueboot
Place bootable ISO files in the /boot/grml folder.
Since this is a system folder, the operation must be conducted as “root”. For example, if the ISO is located in the user's Downloads folder, the command would be:
sudo mv ~/Downloads/<filename.iso> /boot/grml/