$ touch file1 file2
$ tar -cf bla.tar file1
$ tar -rf bla.tar file2
$ tar -tf bla.tar
file2
$

Alternatively:
filexx
filexy
filexz <-- last file added in -c step
tar: Invalid header, starting valid header search.
tar: End of archive volume 1 reached
tar: Unexpected EOF on archive file
tar: Unexpected EOF on archive file
tar: Premature end of file on archive read
       <--- fileyy not displayed.

$ ident `which tar`
/usr/bin/tar:
     $NetBSD: crt0.c,v 1.13 2003/07/26 19:24:27 salo Exp $
     $NetBSD: ar_io.c,v 1.39 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: ar_subs.c,v 1.29 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: buf_subs.c,v 1.23 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: cpio.c,v 1.17 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: file_subs.c,v 1.36 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: ftree.c,v 1.29 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: gen_subs.c,v 1.32 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: getoldopt.c,v 1.19 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: options.c,v 1.69 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: pat_rep.c,v 1.21 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: pax.c,v 1.31 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: sel_subs.c,v 1.19 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: tables.c,v 1.23 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: tar.c,v 1.46 2003/11/24 23:48:47 matt Exp $
     $NetBSD: tty_subs.c,v 1.16 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: spec.c,v 1.53 2003/11/17 00:02:33 dbj Exp $
     $NetBSD: misc.c,v 1.24 2003/08/07 11:25:35 agc Exp $
     $NetBSD: stat_flags.c,v 1.18 2003/10/27 00:12:41 lukem Exp $
     $NetBSD: pack_dev.c,v 1.6 2003/10/27 00:12:42 lukem Exp $
     $NetBSD: rmtlib.c,v 1.19 2003/03/08 07:47:49 lukem Exp $

-sigh- PR needed ? (what is repetively breaking tar -r anyways ?
Since we moved to tar-as-pax it worked, stopped working, was
fixed (hi christos :), stopped working again ...)

-Martin