As the creator of Karate, I strongly recommend you don't do this. In the long term this makes all your projects depend on one common framework - and you should try to reduce the creation of "home grown" frameworks. Especially for a testing framework, you should try not to force teams to depend on an additional library which you need to maintain and version-control.
That said, since Karate can read files from the classpath: you can "ship" a JAR file with common Java classes and even feature or JS files that all your projects can inherit from or "re use". In fact the karate-base.js has been designed to solve for common bootstrap logic or variables / parameters being supplied from a JAR file.
Short Answer: use normal Java techniques (Maven / Gradle) to create a re-usable JAR file. There are multiple ways to use resources (Java, *.feature, JS) from a JAR file. It is up to you how to structure your Maven (or Gradle) projects to make this happen.
EDIT: for those looking for how to create a "runnable" JAR, please see https://stackoverflow.com/a/56553194/143475
