DrizzleZip

DrizzleZip is a tool for Windows (requires Microsoft .NET framework 4.0) written by rain and released in 2013.

Let's say you have a folder with mixed files, some RAR, some ZIP, some 7z and some uncompressed files. Now, you decide for the sake of uniformity that you would like all these files zipped up using 7z, ZIP, Gzip, Tar, Bzip, XZ or even TorrentZip techniques. Normally to do this you would have to jump through a lot of hoops. Enter DrizzleZip, simply select the folder which you wish to unify and click Process and in a matter of no time at all you will be left with a properly compressed directory.

DrizzleZip supports dragging a group of files, or entire folders (up to 1,000 folders) directly onto the application icon for command-line processing. It's also possible to compress each file of an archive to separate archives.

Supported input formats:
 * 7z
 * Zip
 * RAR
 * uncompressed files

Supported output formats:
 * BZip2
 * GZip
 * SevenZip
 * Tar
 * Torrent7z
 * TorrentZip
 * XZ
 * Zip

Below is a screenshot of the main window:



It can be downloaded from here.

Alternative
A good alternative that supports less formats but supports multithreading is Torrentzip.NET.