I imported our whole db (about 60,000 records) but realized that I forgot to flag about 200 or so that are in our "regular followup group".
I'd like to import just that 200 or so but they already exist in our list.
Is there a way to import them into a second List and then merge that list with another and flag the second list emails as a certain segment ("regular followup")?
Re-importing the subscribers to the same list won't be a problem – they will be matched up with the existing email addresses in the subscriber list, and their other fields will be updated according to the latest import.
Is the segment based on a custom field value? If so, importing just those 200 addresses along with that field should do the trick.
In general, "importing someone to a segment" requires that the segment is based on fields rather than actions ("opened a campaign", etc.) since the imported subscribers will to meet the segment criteria to be added to it.