This is supported.

On the Edit Pricing & Delivery page of the offer, under Access & Cancel Settings on Purchases & Subscriptions Page, check the option "Clicking Cancel Link Retains Access Until Expiration Date".

The result will be as described: When a client cancels their subscription for the offer from their Purchases & Subscriptions Page, they will retain access to the subscription until the expiration date of their subscription.

Checking this option applies to all current and future subscribers of an offer for as long as the option is enabled.