@rmschne’s suggestion isn’t incorrect. But you can also do this:
- Decide where the indexed files will reside in the Finder.
- Index that location into your database.
- Move the imported files in the database to that indexed location. By default, they will be moved to the external folder (a process called deconsolidation) and are now indexed.
However, do not do this until you’ve read and understood the In & Out > Importing & Indexing section of the help.