Well, theoretically you can simply decompress JAR files using Java, and then repack the content to a ZIP archive. It is possible, however the reasons for this are questionable.
Convert Compressed archive file package for Java classes and data to WinZIP Compressed ZIP archive.
We have found 4 software records in our database eligible for .jar to .zip file format conversion.
Microsoft Windows software - convert jar to zip on Windows
Popular open source file archiver with a high compression ratio
Convert from jar file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned jar to zip. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
One of the many available file compression utilities
Freeware file compression tool
Apple macOS / Mac OS X software - convert jar to zip on OS X
Full-featured compressed archives manager