There's no one step way of doing this, but this should work for you.
1. Create Staging Table
Create a data extension to hold records of Unsubscribed subscriber records you are going to get with a Query Activity. The fields should look something like the following:
2. Query Activity
Create a query activity to get unsubscribed records from your Publication List changing the ListID below for the ID of your Publication List of interest:
SubscriberKey AS [Subscriber Key],
EmailAddress AS [Email Address],
'Active' AS Status
_ListSubscribers WITH (NOLOCK)
Status = 'Unsubscribed' AND
ListID = 495
3. Data Extract
Create a Data Extract Activity in the Activities tab of Automation Studio. The type of extract you need is a Data Extension Extract. Raise a case with Support, if this is not available within your account.
4. File Transfer
Create a File Transfer Activity in the Activities tab of Automation Studio. Have this "Upload" the file with the name you stipulated in Step 3 to the "ExactTarget Enhanced FTP" File Location.
5. Import Definition
Create an Import Definition in the Activities tab of Automation Studio to Import the file with the name you stipulated in Step 3 from the ExactTarget Enhanced FTP. The target of the import will be your Publication List, the Mapping would by "By Header Row" and the Data Action in this instance would be either "Update Only" or "Add and Update"
Create a new Automation in Automation Studio. Add the items created in 2, 3, 4 and 5. Save and run the Automation.