There is no binary code for J2MEPolish UI library, because these classes are also preprocessed during your application compilation. This is why there is also no build script for J2MEPolish UI part - the resulting binary, if it compiles, would not make any sense :)
If you are using this UI class directly (there is no Polish magic involved) I would suggest to copy it into your own project, make the changes there and you should be just fine.
But there is a small issue. I'm not 100% sure, but Polish build process should be as follows: first your classes are preprocessed and then Polish classes will be preprocessed. If the UI component, that you copied into your project, will stay in the same java package, it could happen that your changes will be overwritten by buggy Polish implementation. You can check it by running the application or looking at the final java code in build directory.
If the overwriting problem happens, then you have two options - move UI class in your project into different package (I'm 99% sure this should be fine if you use it directly) or you need to modify Polish build process, so that it would preprocess Polish classes first and you could overwrite changes done by it.
/JaanusSiim