this error indicates that your ejb bean does not implement DummyBeanLocal interface. As you can see you don't do anything special from jbpm and only use JNDI look up.
So make sure it only casts to the known (implemented) interface of the bean and you should be able to use that from within command.
from my point of view this is a class loader issue.
My ejb is deployed inside of MyApp.war. I add the interace DummyBeanLocal to my jbpm dependencies. On runtime DummyBeanLocal is loaded twice. One time with jbpm class loader and one time with MyApp.ear classloader. Therefore I got a ClassCastException.
How I should use jbpm console to have access to ejb which is still available on my app server?
if jbpm runs withing the same classloader hierarchy as you ejb then you don't need to add dependencies to kjars to be resolved on runtime, meaning you should declare it as provided so jBPM won't load that dependency when creating runtime representation of the kjar.