Official comment
You are not able to. On an occasional, one-off basis, ONTRAPORT's Email Delivery team can change contacts back to BES Single Opt-In so long as you can provide proof that the contact still wants to hear from you.
The contact will also be opted back in if they fill out a webform. To learn more about how to keep your list squeaky clean, check out ONTRAPORT's Active Re-Engagement Campaign.