We can import registries to our app by a CSV file. Our developers handle this on a custom basis. The better you match the format, the easier it will be, however, there is generally a custom charge associated with this import in all cases.
The general layout of the import is each line will represent a single item. However, each line also will have the information of the registry that the item is for. If a registry is to have multiple items, then there will be multiple lines with that registry information, each having different product information.
You can submit one large file with all registry and item information at once. The minimum required fields for the registry are Name and Event date, although there are many optional fields. The required fields for the products are SKU, product_id, variant_id, and wants. Although 'has' can be optionally entered.
Attached is an example file required to import.