How to add custom method to Spring Data JPA

I am looking into Spring Data JPA. Consider the below example where I will get all the crud and finder functionality working by default and if I want to customize a finder then that can be also done easily in the interface itself.

@Transactional(readOnly = true)
public interface AccountRepository extends JpaRepository<Account, Long> {

  @Query("<JPQ statement here>")
  List<Account> findByCustomer(Customer customer);
}

I would like to know how can I add a complete custom method with its implementation for the above AccountRepository? Since its an Interface I cannot implement the method there.