I'm not aweare of any mechanism to achieve that, but you can create more ad-hoc (un structured) processes, what business situation are you trying to model?
I've got a Business Process in which almost eveynode would have transitions to all other nodes. I guess that's what you mean by un-structured process.
I've tried to create a process with "loose" nodes but the parser complains: Node 'Script'  has no connection to the start node.
Exactly, you will probably need to use ad-hoc processes. Take a look at the example files used tests in the jbpm-bpmn2 project. Can you describe a little bit why each node will have a transition to all the other nodes? Cheers