Hello Bhaskar,
Yes I had written the code for the same in PBO & PAI only. In user exit, you are enhancing the code but here you have to add custom fields and write validations, so you need to use PBO & PAI. In PAI you will have to use case to handle different user commands for eg. Save etc.
Revert in case of any clarification.
Thanks & Regards,
Chandni Sharma.