Have you taken a look at the code to see if it's possible? Pretty generic question. Maybe after you take a look at try to implement what you need you can ask specific questions that would be easier to pinpoint the answers to.
You are correect. Also If you see I have mentioned jBPM plugin classes which does the job of setting parameter mapping to BPMN file.
I was able to solve this issue few days back after looking into the eclipse plugin architecture and using WorkItemParameterMappingDialog
After digging more into the source code I found that I can get the process from BPMNModelEditor and using WorkItemNode instance available we can perform various operations.