The only way to add new data (for use in other places, like split nodes, exports, etc.) is by adding it to the information map.
If you just need to get a list of "customers who are in area XYZ", you could write a simple stored process that reads in the external file into a dataset and uploads it to the database for temporary use, then creates a new list of outgoing customer ID's (&outTable) by joining the incoming list of customer IDs (&inTable) with the customer table and with the temporary table from above and a where clause like "where area=XYZ" and you'll have a list you can to continue building your campaign.