No, there is currently no foolproof tool/installer that comfortably installs LibreELEC onto a machine and keeping existing operating systems alive. Perhaps if we had enough man power / skills / time / testing facilities, a capable enough installer could be created. Right now, it still comes down to only a few minutes of manual labor (if you know what you are doing...).
(Mind you, this tutorial is all with no guarantee whatsoever if you yourself fcuk up.)
Since you are using Ubuntu and talk about extended partitions, you should also be comfortable using bootable tools like Gparted Live. If so, create 2 EXT4 partitions : one 512MB system partition, the another one using up the rest of the availabe free space for LibreELEC's storage partition. Whether those are primary or extended is up to you. All primary is possible when you have no seperate partitions for Linux's home, boot, or its other options. So if you have currently only a single EXT4 partition for Ubuntu along with a SWAP partition, it's possible. Otherwise, create the 2 EXT4 partitions in the extended partition area. Also, give the new partitions labels, to identify them more easily.
Reboot the machine, Ubuntu should start.
Go into a terminal. Type sudo blkid
You will get a list of drives/partitions, pick out your two new partitions, and remember their device numbers. You will need them later on.
Now you need to add the LibreELEC menu item to the GRUB2 boot system.
In the terminal, type sudo nano /etc/grub.d/40_custom .
Paste in the following text:
menuentry "LibreELEC 8.0 Krypton" {
set root=(hd0,6)
linux /KERNEL boot=/dev/sda6 disk=/dev/sda7 quiet
}
Replace all three drive numbers (in my case, twice 6 & 7) with the ones you got for your respective system and storage partitions from the blkid command.
CTRL-O for storing the file changes
CTRL-X for leaving the nano editor
Update the GRUB2 menus: sudo update-grub2 .
Reboot your machine and hold the left SHIFT key.
Tadaa !!!
For more convenience, install Grub Customizer or some other grub tool if you want to make the GRUB2 booter visible at boot time.
