Help installing lvm onto an existing Ubuntu 12.04 system

I currently have two 500Gb hard drives.

sda is 500Gb internal drive and I have installed Ubuntu 12.04 desktop on it. sda1 - 20Gb root partition sda7 80Gb - home partition sda5 - 365Gb unused.
sdb is external (USB) 500Gb hard drive sdb1 - I use as a home file server storage and share using samba.

What I would like to do is add the sdb1 and sda5 partitions into a LVM Volume Group and that would become my Logical Volume for file server storage. That would increase my file server storage

So my question is, can I change my existing file system into a LVM system without doing a new installation ?

I don't think that it is possible to change the file-system without re-installing Ubuntu or for that matter any OS.