LVM doesn't work on a slice philosophy the way Solaris did. It works on a PV > VG > LV system, where PV is physical volume (a disk or partition), VG is a volume group, the logical construct of one or more PVs, and LV is a section of a VG, made up of extents.
You can add multiple PVs into a VG, even migrating old PVs out and adding new ones in, moving the data around. However, you can't add the existing root volume to a VG without losing the data, as technically, the partition type changes and the data is not encapsulated.
You CAN try to migrate to LVM, but it is not fool proof and if something goes wrong, the data can be destroyed. It is not as safe as I would like to recommend.
http://www.linuxweblog.com/blogs/san...filesystem-lvm
http://tldp.org/HOWTO/LVM-HOWTO/upgraderoottolvm.html