Hey @Kim30
Renars here from Matrixify app.
Thank you for mentioning us.
As far as we know, there is no way to set any created_at dates in Shopify. The same applies to Orders, Fulfillments, Transactions, products, Customers, and any other details that have created_at dates.
Other entities, such as Orders and Products have different date fields (processed_at for Orders and published_at for Products) that you can set to a past date and see in the admin, but as far as Shopify documentation shows and we know - Customers do not have such a date.
Just another possibility to look into - if you using Matrixify app also for your reports, if so then maybe you can look into importing Metafield for the customers where you store the created date if its created in the past? Then when exporting Customers with Matrixify, you can also export their Metafields.
I hope this gives some more insight into the Matrixify app and Shopify API.
Hopefully, you can find a solution that works best for you. If you need any Matrixify related assistance - let us know!