The customer import has a DefaultPaymentMethod column which you can add Stock|RewardPoints. However the customer payment methods API seems to only deal with credit cards.

First, query a user with a credit card to ensure it's working: /api/v1/customer_payment_methods?customer_id=<customer_id_here>

Now repeat the same request with a different user ID and ensure in the admin panel that their default payment type is reward points. Their default payment type of RewardPoints doesn't show up. I don't think this works with custom payment types either but I haven't tested.

I'd expect similar functionality to the Customer Import's DefaultPaymentMethod, probably in the customer endpoint