If you are used to grub, you're likely to be seriously disappointed in lilo.
I prefer to always use a separate /boot partition, so deleting a distro doesn't cause the problem you are fearing. You might like to consider it to save future repetitions.
As to your question, I'd suggest you build the (separate) /boot, and (from current RH9) install grub there, and copy your current grub.conf over - should work with minimal changes.
If you *reallly* want to use lilo, you probably could run liloconfig from your current Slack install, and direct it to the MBR. Will presumably need some updates to /etc/lilo.conf (???) to handle the multi-boot properly.
Head over to the Slackware forum - quick search should give you some hints.