Assume 4 CM Lists:
ListA B C
I'm guessing that if I exported CM list to CSV I could then add a field (LiastA set to True) then when I import into my MainList then they'd have a field which would tell me where they came from, right?
And I could use that as a Segment, right?
Is there an easier way to do that without exporting to .csv (or excel, etc.) ?
I think you've come up with the best solution there MrAnalogy; you need a way to know who was on each list, and a custom field is the way to go. Exporting and updating is the best way to go about that.