Hi Nitin,
You can automate this via XML Import sets. Please refer below link with the exact steps that you need to follow in order to automate this. In the last step, you will have a Transform script(mentioned in point no 4 in the below link) which will copy the sys_id of new records from the source to the target instance.
https://docs.servicenow.com/bundle/london-platform-administration/page/administer/import-sets/concept/c_XMLImportSets.html
-Pradeep Sharma
