Hi @AtrayeeDutta ,
For your use case, you can use "modifyuserdatajson" during the import process.
You can also make use of the "savforsav" connection.
You would be able to use the "modifyuserdatajson" under HR Connector, and apart from that their is not way you would be able to generate the systemusername for the existing user.
For updating the email address you can use the "userupdaterule" for the email regeneration for the existing user.