Hi Shiva,
I hope you might have created aet fields in BUT000 table itself. There is one option you can fill aet fields using direct but000 table update using modify sql statement.
When you create BP using BAPI_BUPA_CREATE_FROM_DATA you will be receiving the partner number created from importing parameter BUSINESSPARTNER. You can use this value and update the corresponding record in but000 table with aet field values. You just need to fetch the custom fields from your file and update them using partner number generated by above FM in table BUT000. This is just an idea, there might be some other options as well.
Regards,
Dharmakasi.