Can I add methods to a stub for a Durable Object?

I would like to do sth like


where customer is a stub for a Durable Object. And totalRevenue() would internally (i.e. transparent for the caller) send the request to determine the total revenue to the Durable Object connected with the stub.

I must admit that I am no JS crack (rather to the contrary). So maybe my question has a trivial answer.

Thx in advance