Modify an Oracle Dataguard destination database
If you setup a logical standby, then you can change the data in the logical standby. You will probably have trouble supporting the kind of change that you seem to want. Have you thought about talking to the vendor to see if they can add the logic that you want to the product you pay them for? If the vendor makes the change, then they will/should support it. You can create materialized views in a different schema and add whatever functionality that you want to the materialized views. You then just need to refresh them when needed.