Mitch asked a while ago if it's possible to use the "Import from a file" feature to modify existing subscribers' multi-option field, e.g., to update some subscribers with an additional option?
E.g., I am consolidating lists of contacts into CM, but want to track the list sources. I upload 'Contacts 1.csv' and 'Contacts 2.csv'. If I have a multi-option custom field, 'Sources', that can handle values of '1' and '2' and a corresponding column in my CSV files, how can I upload both, confident that any overlap between the groups won't be simply over-written by the second upload?
They would be overwritten if you have the same address with two different values - you'd need to combine the lists outside of Campaign Monitor, so you are importing one file with the final set of data.
Campaign Monitor will overwrite whatever is in the custom fields with the data you import.