Activiti aims to be a BPMN-2.0-compliant Engine. So when you export valid BPMN-2.0-XML from a third party tool such as Signavio Activiti should be able to parces the proces definitions without problems - as long as only supported elements are used.
Since it can deal with several pools within one process definition file now and since it can distinguish between executable and non-executable pools parsing a process definition file with Diagram Interchange information should not raise an error. This is currently the case. Removing the DI information from the process definition file solves the problem. But this is not desired as you then lose the graphical representation of the sequences within your pools.
The DI parser should be able to handle collaboration between two or more pools. The first step would be to simple ignore the information. The second step would be to render the collaboration (message flows, pools).
Further information can also be found in the forums: http://forums.activiti.org/en/viewtopic.php?f=6&t=3512
I am going to supply a testcase and maybe also a patch for the first step.