Sorry to say the extension is not supported by CA as part of the product software.
When you say you are unable to build the JAR, do you mean it contains compile errors that prevent the build or is there a different issue?
Your challenge / experience with custom JARs brings up an important point for everyone to consider.
Often custom JARs leverage DevTest framework components and classes as well as 3rd party JARs.
As DevTest product improvements occur, DevTest classes may change without notice including moving from one package to another. Similarly, the 3rd party, open source JARs on which some of the older extensions are built may also change or go out of support.
I believe the extension you are referring to was generated circa 2015 which likely means it was built on a combination of Java 7 and possibly an older version of DevTest (maybe v7.5). This may or may not be a reason for build issues. Lastly, I would verify that any 3rd party JARs used by the extension are still viable as they may or may not use Java 8 compatible features.