Both JAR and CAB files are basically archives so you could actually convert them between each other. However this will just repack the files and nothing more. In case of installation packages for mobile devices they will no longer work. You would need a dedicated converter for that.
Convert Compressed archive file package for Java classes and data to Windows Cabinet Compressed Archive.
We have found 3 software records in our database eligible for .jar to .cab file format conversion.
Microsoft Windows software - convert jar to cab 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 cab. 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.
Freeware file compression tool
One of the many available file compression utilities