I created a copy of the tar archive and copied to entire content to it. Then I delete the old tar archive.

public void appendFileInTarArchive(String tarPath, String tarFileName, String file2WriteName, String file2WriteContent) throws AuthenticationException, IOException {
if (tarPath == null || tarFileName == null || tarFileName.isEmpty()) {
LOG.warn("The path or the name of the tar archive is null or empty.");
return;
}
final File tarFile = new File(tarPath, tarFileName);
final File fileToAdd = new File(tarPath, file2WriteName);
FileUtils.write(fileToAdd, file2WriteContent);

if (file2WriteName == null || file2WriteName.isEmpty()) {
LOG.warn("The name of the file to append in the archive is null or empty.");
return;
}

ArchiveStreamFactory asf = new ArchiveStreamFactory();

File tempFile = new File(tarPath, "tmpTar.tar");
tempFile.createNewFile();

try {
FileInputStream fis = new FileInputStream(tarFile);
ArchiveInputStream ais = asf.createArchiveInputStream(ArchiveStreamFactory.TAR, fis);

FileOutputStream fos = new FileOutputStream(tempFile);
ArchiveOutputStream aos = asf.createArchiveOutputStream(ArchiveStreamFactory.TAR, fos);

// copy the existing entries
ArchiveEntry nextEntry;
while ((nextEntry = ais.getNextEntry()) != null) {
aos.putArchiveEntry(nextEntry);
IOUtils.copy(ais, aos);
aos.closeArchiveEntry();
}

// create the new entry
TarArchiveEntry entry = new TarArchiveEntry(file2WriteName);
entry.setSize(fileToAdd.length());
aos.putArchiveEntry(entry);
IOUtils.copy(new FileInputStream(fileToAdd), aos);
aos.closeArchiveEntry();

aos.finish();

ais.close();
aos.close();

// copies the new file over the old
tarFile.delete();
tempFile.renameTo(tarFile);

} catch (ArchiveException e) {
LOG.error(e.getMessage(), e);
} catch (IOException e) {
LOG.error(e.getMessage(), e);
} finally {
FileUtils.deleteQuietly(fileToAdd);
}
}