For each ZealOS partition, add lines to /etc/grub.d/40_custom
in this format:
menuentry "ZealOS" {
set root=(hd0,msdos6)
chainloader +1
}
In the above example, ZealOS is installed to hd0,msdos6
: the 6th partition of the first hard drive.
Run sudo update-grub
to update the auto-generated grub configuration with the custom entries.