Currently the Feeds module will not update an existing target node's multi-value fields by adding a value to the field.
You import some nodes using feeds with a guid for each node. You then want to import another dataset, which has the same node guids, with some new values for a target multi-value field. You want your new feed pass to update existing nodes and add values from the new dataset as additional values on your multi-value field.
Currently feeds will overwrite all old values in the target field with new values from the new dataset. You don't want that! You want the option to append values from the new dataset to the existing values already in your target field.
If anyone knows tricks to achieve this, please speak up.