You'll need to mount the partition where Suse's kernel is, then edit Slackware's lilo.conf and reinstall LILO.
You'll need to do something similar to this:
# mkdir /mnt/suse1
# mkdir /mnt/suse3
# mount /dev/hda1 /mnt/suse1
# mount /dev/hda3 /mnt/suse3
Now, get into /mnt/suse1 and /mnt/suse3 and check where the kernel is, usually under /boot/vmlinuz, but may be /vmlinuz
Once you find it, edit /etc/lilo.conf, add a similar entry to slackware's, but change image= to where suse's kernel is (including /mnt/suseX) and root= to your suse's root partition (hda1 or hda3).
/sbin/lilo -v -v -v
That should've done it.