As long as they're not recurring subscriptions you can extend all their expiration dates manually within the _cbsubs_subscriptions database table. Example as follows.
Code:
UPDATE `jos_cbsubs_subscriptions` SET DATE_ADD( `expiry_date`, INTERVAL 15 DAY ) WHERE `status` = 'A'
If they are recurring subscriptions then there is no means of adding time to their subscription and best I can suggest is provide them a coupon for a maybe 10% off their next renewal.