NetBeans Module development, how to use an external jar library.

Hi, I'm developing a NetBeans module to have a customized Java hint but I have some problem to use the Java library Java\jdk8\libs\tools.jar that I need to parse the code.
In particular I need to use the com.sun.tools.javac.tree.JCTree.JCAssignOp class to manage the cases in which the code contains the  +=    -=    *=    /=   assignment operators.
This library can not be added to my Netbeans module project using ProjectProperties -> Libraries -> Module Dependencies (or at least I did't find it) and if I add it using ProjectProperties -> Libraries -> Wrapped JARs I can compile and run my module but it seems that the lib is loaded using a different ClassLoader so I have a ClassCastException when I try to cast a JCTree also if the cast is valid (and JCTree objects are not recognized when I use the instanceof operator).

Could you kindly give me any hint about how I could solve this issue?

Thanks in advance,
best regards.