As you are writing on the .NET Adapter API forum,
it is unclear whether your are working with normal in-process java Adapters or with Remote Adapters.

Assuming that you are using java adapters and that they access external libraries through some jar files,
I confirm that any jar that is put in the adapter's lib folder or in the shared/lib folder is added to the classpath available to Adapter code;
similarly, any class or other resource file that is put in the adapter's classes folder or in the shared/classes folder is available to Adapter code.

You can ensure that the classes and resources that you placed in these folders are available by trying to load them (just for testing purpose) from inside your Adapter init method.
If they still can't be seen by your external library, then try adding them to the global classpath, by modifying the JVM command line in the launch script (namely, LS.bat or LS.sh).