Hello Pankaj,
If you want to change the values of the fields on some logic, you may use BBP_DOC_CHANGE_BADI.
If you want to validate the data in the fields you can use BBP_DOC_CHECK_BADI.
These custom fields are added to BBP_PDHSC table for header. Hence you can check if the values of the fields are there in this table.
Regards,
Yayati