I've been thinking about this and it's not that easy. My suggestion would be do do it stepwise and using "standard" procedures:
1. partition your harddisk into 4 (swap, /boot (~100MB), / (~3GB) , and an LVM partition for the rest of the disk)
2. install a stock Slackware with kernel 2.4 using the first 3 partitions
3. upgrade to kernel 2.6 and LVM2
4. format the LVM partition and create LVs as you require
5. copy the / partition to the LVs (maybe it's best to do this with a life CD) and change fstab and lilo accordingly. For lilo, you'll have to chroot into the LVM partition
6. reboot into your LVM partition
7. if that worked well, convert the old / parition into a PV and add it to your LV.
This is all purely theoretical. I'm interested in trying but first I'd like to know if any of the gurus here have a better way.