Oracle Database Discussions

Hi

Can I apply index on the column(s) of external table.
If yes then how?
If no then please guide me how can I achieve my following goal.

Despite deleting old data & managing growing datafiles I want that I use the read only .dat files and replace the old/outdated .dat files.
It is easily possible with external table utility but I also need indexing to enhance performance.
Can anyone help me?

Thanks
Jawad