Hi @nitish_kumar,
Data migration is a very difficult task.
Most users run two installations in parallel, start new process instance only in the Camunda engine and run pega until all processes are completed and can be switched off.
You can also use the process instance modification API to start process instances in Camunda somewhere else than in the start event: Process Instance Modification | docs.camunda.org. Here you have provide all required process variables.
Hope this helps, Ingo