Well, you are a paid customer anyway
Creating new properties file is the only way and probably the best way.
I suggest you get the existing properties files from the SVN of JCL, add your own properties files, then modify your build script to make these properties into a jar. The reason we use a separate properties jar, just so you know, is because there are so many properties files and it will increase the size of JCL jar. A large percentage of customers need English only version so increasing the jar size is not fair for them.