With SD 8.5
From the release notes
To extend a current process in IM / CM etc the following Article has been written.
How to add custom data to existing processes like Incident Management, Change Management etc
But the part to join your new custom data to the original ticket hasn't been documented.
Since you have extended your class to "RelationalMappingProcessData" there will be a
This is where you take the SessionID from your Incident/Change and map to the new Data Type.
You could create a Workflow that passes in the SessionId as an input.
This calls
The maps using
Map from your "SessionId" to "WorkflowTrackingId"
This will make the join between your two tables and be the association needed.