Is it possible to add a password to an existing zipfile with 7zip without going to all the trouble of unpacking it and re-packing it again?
You can password-protect an existing ZIP file with zipcloak.
EDIT: T. Furukawa crafted a patch for zipcloak that adds a password option, so batch processing is much easier:
C:\>for %f in (*.zip) do zipcloak -p password %f.
If you prefer a GUI, both WinRAR and PeaZip can batch encrypt existing archives.
In WinRAR: Select ZIP files > Tools > Convert archives > Compression... > Set password...
In PeaZip: Select ZIP files > Convert > Enter password / keyfile (optionally set algorithm to ZipCrypto under the Advanced tab for compatibility with Windows' built-in ZIP handling)