There are a couple of ways to go about that...
You may want to try the xmlimport feature, where you would have to create a rather complicated XML file for each issue, and run the xmlimport.php file to update your database.
You may also try to use the system to publish previous issues but you would have to login as the author of each article, and perform all author steps, then either select an issue and change the numbering, or create a previous issue manually in the database for you. You MUST login as the author because the system uses the logged user's id, or you will have to manually change the id code which is rather boring to do.
I hope the PKP team will allow the Editorial Team to possess total permissions for publishing, which would allow them to publish for an author, as well as fix the Metadata area, where we cannot add forgotten co-authors.