Finally it wasn't' necessary to use Javassist. With Java Reflect was enough, in this way:
String tempClassPath = tempDirPath + serviceName + sbbJarCmpt;
Where tempClassPath is the Path location of the service .jar file.
Now, with Java Reflect:
URL[] classes = {new File(tempClassPath).toURI().toURL()};
URLClassLoader child = new URLClassLoader (classes, this.getClass().getClassLoader());
Class fieldClass = Class.forName(className, true, child);
With this, I get an instance of the class and I can continue with the rest of the process.