Parties to any policy of life insurance now or hereafter issued shall have the right at any time to mutually agree, in writing, for an extension of time in which to pay a second or subsequent premium on the policy, upon condition that the failure to pay the amount agreed upon at the time agreed, shall lapse the policy as of the date mutually agreed upon in the writing; provided, no such agreement shall impair any right to extended or paid-up insurance which the insured may have under the policy, nor any right to have the premiums, any part thereof, or the amount payable for the extension charged against the policy under the terms of the policy. No such agreement need be attached to or made a part of the insurance policy so affected.