Could you elaborate on the question. Is the question about recovering expired data? or setting expiration on existing data?
Data that have expired are no longer indexed in the database and will not be retrievable. That said, if you are using persistent storage, and you desperately need to recover data that were accidentally set to the wrong expiration TTL and have already expired, you may be able to change the time on your server to a time and date prior to that expiration and do a cold-start to re-index data from disk.
A few caveats with this approach would be that you could only recover objects that have not been overwritten. The server time would need to be changed on the entire cluster and also No client should be hitting that cluster to prevent new writes or update to occur during this maintenance.
Here is some additional iinfo on Expirations: