Possible to add an LVM to existing Wheezy box

[Ron Leach <ronleach@tesco.net>]

Is it possible to create an LVM on a machine that's already running 
Wheezy but without an LVM?

I've looked at
  https://wiki.debian.org/LVM
and the howto which the wiki refers to,
  http://www.tldp.org/HOWTO/LVM-HOWTO/initdisks.html
but these seem, to me, to be meant for an initial install or, at 
minimum, for empty disks or partitions.  Whereas I wanted to create an 
LVM from combining an existing 1.7TB partition (preserving its data) 
with a new, empty, 2TB disk, so as to have a 3.7TB logical volume that 
already contains the 1.7TB's worth of data.

If it is possible, the step I don't understand is how to create an LVM 
physical volume of the existing 1.7TB partition, without destroying 
the data already on that partition.  The howto suggests that pvcreate 
should be used but won't that damage the data and filesystem on that 
partition?

Apologies if my question isn't clear.  Basically,
(i) can I make an LVM based on an existing partition while preserving 
its data and, if so,
(ii) how do I avoid damaging the data on that partition?

regards, Ron