Hi Felix,
I guess the key of problem is What you really want to do? You never tell us that, just ask where is the ActionHandler, how could we use ActionHandler to let process go on execute. If we still focus on how to implement a operation, I am afraid we could never find a solution.
From your description, I think you would better use TaskActivity to replace StateActivity. Because in fact, what you want to do is let a document or a contract or something else to be reviewed, to be researched, to be completed or to be closed. There must have someone to do these jobs. So we could use TaskActivity to create a task to these people and let process waiting for people operations. When the people complete his task, related process will be signal to the next step.