Help with Data Load Mapping for Transaction Matching Files

Summary

How can you limit records imported to a specific set of accounts?

Content

I have activity files to import for Transaction Matching, but want only specific accounts to be imported. In Data Load Mapping I can ignore records for specific accounts, but I can't work out how to "not ignore" the accounts I do want.  In other words if the record/row is one of the accounts I do want import it -  and if not ignore the record/do not import.

Any help would be greatly appreciated! I am at a standstill.