After importing activities, is it possible to update them using the import tool? We want to "fill" data in fields that are empty.
This is possible in Contacts using the External ID field (among others). In the import of activities there is a field for External identifier, but I got an error when I tried to use it. Instead, I created a custom field to identify the activities imported. The Activity Import tool doesn't seem to have a way to detect duplicates and fill data instead of creating a new activity. The documentation doesn't mention anything about updating activities.
EDIT: In case it helps, when mapping imported data to "External identifier", the import reports this problem on the rows using it: "No Matching Contact for External ID:" (and the ID used). This gives the impression that the importer is looking for activities with that external ID, and perhaps if there would be a match an update would be possible? However, if it is not possible to set an External ID, it won't be possible to match it with new imports...