What's the most efficient way to temporarily 'tag' records for reporting purposes?

Summary: We need to check on a particular field for a large group of contacts, but have no way of querying for them. There is a separate/external and 'official' contact database that we have, from which we can draw a list of records - the catch is that the piece of information we want *about* each of those records is in our B2C Service site. Is there a (more efficient) way other than creating a custom contact field and simply calling it 'Tag' or something, so that we can create a default value when we do an import? I'm assuming