You may have already sorted this out, but best approach for updating existing contacts via a CSV file is to first export the contacts from Infusionsoft into a CSV file, open the file in Excel, append the tag data in a new column or an existing one designated for tags, import into Infusionsoft.
IMPORTANT: the CSV file exported from Infusionsoft will have an “ID” column - this is the unique identifier for that record within Infusionsoft. Be sure not to modify or delete the ID or sort the data without the corresponding ID or you will end up with mismatched records.