if you need to compress Zimbra directory with tar, you just need a command line like:
-j ... compress with bzip2
tar -jcvf /tmp/zimbraarchive.tar.bz2 /opt/zimbra
-c ... create
-v ... verbose, prints each file processed, but you can miss some errors in the output (optional)
-f <file> ... name of archive file
Adjust the names and paths for your needs.
An archive like this can then be restored (into current working directory) with
-x ... extract
tar -jxvpf /tmp/zimbraarchive.tar.bz2
-p ... preserve permissions when extracting
Ofcourse Zimbra has to be down to be able to archive a consistent and restorable state. Or use some kind of volume snapshoting for example with LVM, but that's a bit advanced stuff.
Hope this helps.