[Bug-tar] appending a directory structure to an existing archive

[J.P. Trosclair]

I'm trying to append a directory structure to an existing tar archive. The
following works for creating an archive without any problems:

tar cfp test.tar --no-recursion --from-files <(find /var/lib/postfix
/var/spool/postfix -type d)

If I add the A or --catenate option then I receive an error:

tar pf test.tar --catenate --no-recursion --files-from <(find
/var/lib/postfix/ /var/spool/postfix/ -type d)
tar: /var/lib/postfix: Read error at byte 0, while reading 4096 bytes: Is a
directory
tar: Error is not recoverable: exiting now

Any ideas or am I barking up the wrong tree here? If it's not possible I
can think of other means to the same end, this just seems like the most
efficient method for what I'm looking to do.

Thanks