It's actually quite easy:
create 2 partitions, a 512MB (better 1GB) FAT partition (for "boot/flash") and another one, some 5-10GB (or larger, depending on your needs) ext4 one (for "disk/storage")
Then copy everything from the FAT partition of the LE USB thumbdrive to the FAT partition on your harddisk / ssd.
Now run "blkid" (as root) and note the UUID values of your boot/flash and disk/storage partitions.
Then edit /etc/grub.d/40_custom and add a block like this - replace the UUIDs of the "--set=root ..." and "boot=UUID=..." entries with the one from your boot/flash FAT partition and the "disk=UUID=..." one with the one of your ext4 storage partition
menuentry "LibreELEC" {
insmod part_gpt
insmod fat
search --no-floppy --fs-uuid --set=root 2DBA-D3CB
linux /KERNEL boot=UUID=2DBA-D3CB disk=UUID=191250c6-b51d-4205-8832-c40a18ca24da quiet
}
now run "update-grub" and enjoy.
so long,
Hias