Hi Jean-Noel,
I think there shouldn't be any issues with using both the BPMN 2.0 editor as the jBPM 4 GPD in one Eclipse installation. I did not test it though. I also didn't test whether the BPMN 2.0 editor works in Ganymede. It was developed against Helios.
As for a migration tool, Eric Shabell and team are working on a project to migrate jBPM 3 to jBPM 5. Talk to them if you want to include the migration from jBPM 4. Also, if you just want to translate the jPDL files to BPMN it should be possible with an XSLT transformation.
Cheers,
Koen