So edit your /etc/lilo.conf file and add an extra stanza similar to the one below:
image = /mnt/tmp/boot/vmlinuz
root = /dev/hdb5
label = RedHat
append = "splash=silent resume=/dev/hdb5"
initrd = /mnt/tmp/boot/initrd
But, you need to make sure "root" is the appropriate partition, the image file is the correct one for whatever distro you are loading and change that label line also.
Then as root in a terminal you need to mount the partition where the distro kernel is kept and then run lilo:
mount /dev/hdb5 /mnt/tmp