What you are looking for is encrypting a drive "in place". Some software packages do provide this, however you can also do this with Lion's built-in whole disk encryption, which you have to enable via the terminal, since the diskutility does not expose this functionality. It was intended only to encrypt the main OS disk in place, but is available if you need to do it by hand for other volumes.
Instructions can be found here:
To convert an arbitrary volume to Lion’s whole-disk encryption, you
use diskutil’s core storage “convert” command, and provide a
passphrase. For example, if you have a volume called “Data” attached
to your Mac, you would run something like this from the command line:
% diskutil cs convert /Volumes/Data -passphrase [yourPasswordHere]
What this does is kicks off a conversion process similar to what the
System Preferences panel does when allowing you to convert your main
startup volume to core storage with encryption. At any time during the
conversion, you can use the diskutil command again to see status of
your volumes, whether they are encrypted, not encrypted, or
in-progress while converting.
% diskutil cs list
You’ll see a bunch of information, but search carefully for the named
volume (e.g. “Data”) that you just started the conversion process on.
You’ll find a line starting with something like:
This shows you what the progress in the conversion is. From time to
time, check this manually, to see how far along things have
I'm not sure that this will work on a network volume. If not, check out TruCrypt carefully. It allows in place encryption of non-system windows volumes, and so it may have the same functionality for OS X volumes. Otherwise, the google search (first link in this answer) does bring up some other options.