Hi SL,
You can do this in a microflow by retrieving all of your archive objects, using an iterator to iterate them, retrieving your associated object, and then using the change object to set the association. You can add a microflow button on a page to trigger this microflow, or you can run the logic after your flat file import.
Here is a quick example of associating a list of objects to a single object.
Here is the model share if the screenshot gets cut off.
https://modelshare.mendix.com/models/4028a39a-0052-4627-8a11-8e0b87e28014/example
Hope this helps!
Thanks Austin! On the second Retrieve, it says “list of” instead of just the object name; does that matter? I also get an error saying a value is required on the Change action/