Is it possible to import orders into Craft Commerce?
Sprout Import supports importing Craft Commerce Orders on Craft 3.

I can't speak to which fields are required off of the top of my head but importing to custom fields, variants, and even custom fields like Matrix on Variant Layouts is possible. Any required fields you don't include will throw an error and tell you they are needed.

An example of the Orders import format is available in the Element Importer docs.

Sprout Import uses a specific JSON format to perform imports so you will need to convert your CSV export into the appropriate JSON format to import if you choose to go this route.