Você pode fazer isso usando bsdtar :
ire@localhost: bsdtar -cvf pax.tar --format=pax @gnu.tar
ire@localhost:file gnu.tar
gnu.tar: POSIX tar archive (GNU)
ire@localhost:file pax.tar
pax.tar: POSIX tar archive
@archive
é a opção mágica. Na página de manual :
@archive
(c and r mode only) The specified archive is opened and the
entries in it will be appended to the current archive. As a sim-
ple example,
tar -c -f - newfile @original.tar
writes a new archive to standard output containing a file newfile
and all of the entries from original.tar. In contrast,
tar -c -f - newfile original.tar
creates a new archive with only two entries. Similarly,
tar -czf - --format pax @-
reads an archive from standard input (whose format will be deter-
mined automatically) and converts it into a gzip-compressed pax-
format archive on stdout. In this way, tar can be used to con-
vert archives from one format to another.