How to create a custom recipient property
Navigate to your account settings and select Recipient Properties from the menu on the left side of the page.
Select Create Property on the top right of the page to create a new custom property.
In the Type dropdown, select if your new field will be a Date Field or Text Field.
Name your new property and select Create Property.
Once your new property is created, it will be available in the field mapping when creating a new recipient or importing a recipient list. For more info on creating recipients, please refer to this article How to create recipients.