Is there a way to preserve existing data when importing into multiselect and checkbox fields?
Depends on how you define non-manual. Only non-technical I know of is


import just the cid, add to group as part of import, then export
do a vlookup, and get your new data and old data for those fields on same spreadsheet
use CONCATENATE to combine your new 'option values' with the existing 'option values'
do import