Both war and jar are ZIPs. Thus, they can be easily converted.
However, wars usually store webs, while jars contain desktop programs.
So, converting a war to a jar would not be useful because, as the war contained a web, the converted jar will also contain a web, so trying to run it (as a program) will cause an error.
If you're looking to convert a web (the war) into a desktop app, this is not the way.
Please, take a look at Prism.