Is it possible to compose policies in the same way that I'd be able to compose AWS IAM policies? Let's say that I've got a policy that allows creating invoices and a policy that allows sending a payment request. And I want to create a policy for the financial admin that is simply referring to these 2 policies?

Not at the moment, I am afraid. Composition/inheritance setups tend to have subtle edge cases and hard to debug issues so we are cautious about introducing those features. A little duplication is preferred over complicated dependency chains.