So here is what I did so far:
1. Installed LibreELEC to the hard drive using the standard Installer
2. Installed volumio to another drive
3. mounted the volumioimg and volumio_data partitions and backed up all the files (incl. hidden, system, everything). I also got the contents of volumioboot partition, but since it seems a standard syslinux boot partition (fat32) I not used it for the final compilation, just examined what is inside of syslinux.cfg for volumio and I reused this information to expand the syslinux.cfg of LibreELEC.
4. Resized the "Storage" partition on the hard drive of LibreELEC.
5. Created two ex4 partitions using the free space.
6. Copied the previously backed up volumio files to the two new partitions accordingly
7. Mounted LibreELEC "System" partition and updated syslinux.cfg using the information I extracted from volumioboot partition.
The problem is that it is not working. The NUC is keep booting to LibreELEC and the boot menu is not appearing at all. Here is the contents of my edited syslinux.cfg. Can you please tell me what did I missed?
TIMEOUT 600
MENU TITLE Mediacenter
DEFAULT 1
LABEL 1
MENU LABEL LibreELEC
KERNEL /KERNEL
APPEND boot=LABEL=System disk=LABEL=Storage quiet
LABEL 2
MENU LABEL volumio
LINUX vmlinuz-4.16.0
APPEND ro imgpart=UUID=8999785f-8797-4b70-90a5-ea8e8046ca6a bootpart=UUID=93C6-C120 imgfile=volumio_current.sqsh quiet splash plymouth.ignore-serial-consoles vt.global_cursor_default=0 loglevel=0 USE_KMSG=no
INITRD volumio.initrd
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Of course I updated the imgpart=UUID for volumio accordingly (I checked the new UUID of the partition in gparted), although I don't know what bootpart=UUID is for, so I did not touched it. But as I mentioned the problem is that the menu does not appear at all.