Is there a standard way to import activities with an external identfier in the same way as contacts to help us identifiy what has been previously imported.
otherwise, I guess we would just have to create a custom field.
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At first, I thought you meant importing activities and joining them to contacts with the contacts' external IDs. This is the most common way I do activity imports.
However, reading closer, it sounds like you want to have an external ID field for the activity. You'll need to create a custom field for this. Also, you should remember that your custom field will not do the same magic as the contact external ID field: nothing will enforce that the value is unique, and nothing will automatically match activities based upon this field.
It still might be useful for cross-referencing your imported activities with the source, but you'll have to apply your own logic to the field.