jar xvf /Applications/NMRViewJ.app/Contents/Resources/app/lib/dcengine-9.2.0-b4.jar reslib_iu
Similar protocol on Linux or Windows, but the path to the "lib" directory containing the jar files will be different.
Note, jar is a command which is installed with the JDK (java development kit), and may not be present on your system. But a .jar file is just a .zip file with a particular layout so you could also use unzip (or other unzipping program).
Once you create a new .prf file you can place it in a directory which will be searched prior to the default residue library.
I'll send info on that in a subsequent post.
Bruce