Like I said. If you use expdp, then your ONLY tool is impdp. You can't use external tables directly. You can force external tables from data pump, but you are using impdp.
To force external tables add
To your impdp command.
create table external
as select * from table-name;
The .dmp file here can only be load with external table option, and not with impdp. Is that right?