Hi Alessandro,
you have to use a classloader to instrument classes.
If you use an agent on your jboss process you can control the main classloader and then every class loaded by jboss.
You can call transform method in preMain method of your agent.
If you want to monitor classes and method and also add actions you can use byteman. It helps you to registering and unregistering rules (instrumentations) dynamically to your classes.
Ciao