Populate custom fields with a mass import (contacts)?

Populate custom fields with a mass import (contacts)?

I have created custom fields and I want to mark 500 of my 785 records in one of the fields.  I do NOT want to import the 500 records again and have to de-duplicate all of those records.

How can I do this?  Is it even possible?

I want a segment of my total database invited to an event and I need those records marked that the contact was invited.