Hawaii Revised Statutes Division 2. Business § 431:10-220

(a) No agreement in conflict with, modifying, or extending any contract of insurance shall be valid unless in writing and made a part of the policy.

(b) No insurer or its representatives shall make any insurance contract or agreement relative thereto that is not plainly expressed in the policy.

(c) The requirements of this section shall not apply to the granting of additional benefits to all policyholders of the insurer, or a class or classes of them, which do not require increases in premium rates or reduction or restrictions of coverage.