Hello Singh,
i personally wrote my own UME interface & registration as UCES is using the Portal environment and the integrated registration process is only working for the ABAP scenarios.
But creating within the UME user records is not a big challange, just replace the newUser logic within the LoginAction to call the UME interface rather then doing a backend call.
But avoiding the licence costs this way is not smart, SAP simply does look into your UME instead of the ABAP system. It might be smarter to contact your Account Manager and discuss a realignment of the licence.
Regards,
Andreas