(1) By mounting it on a loop back device to expand the filing system at a terminal or a ready-made tool from a desktop, like to open as archive in Ubuntu.
(2) Copy the files to a partition.
(3) Replace the isolinux boot loader, which boots only from a CD or DVD, with a hard disk based boot loader like Grub or syslinux.
The above steps works satisfactorily. However not all distro iso have been arranged bootable from a hard disk. Many could fail because the installer could keep going back to to find the files from a cdrom.
More details in here
with examples. The thread was originally written to put several iso into one DVD and then later transferring the lot into a USB jump drive.