Well, I think what he might be asking is, if he's got /home on a separate partition, say, can he install Slack and keep /home. And, yeah - just don't format it - say Mandrake root and swap and home are /dev/hda5,6,7. Just install Slack to hda5 (you do need to format/erase and start fresh on that one - like Skyline says, you can't 'upgrade') and don't mess with 6 & 7 except to mount them into the Slack root filesystem. I can't remember the details but it'll be pretty clear what to do once you start installing - just remember that you've already got stuff partitioned and want to preserve most of what you've got.
Flipside is I'd expect a lot of your personal configs will be broken but it's a lot easier to adjust part than having to recreate all.