A life insurance company may enter into subsequent agreements in writing with the insured, which need not be attached to the policy, to extend the time for the payment of any premium, or part thereof, upon condition that failure to comply with the terms of such agreement shall lapse the policy as provided in said agreement. Subject to such lien as may be created to secure any indebtedness contracted by the insured in consideration of such extension, said agreement shall not impair any right existing under the policy.