To convert public folders to a mailbox, you would simply click on all folders in Outlook, click the top-level public folder and then export that folder to a PST, ensuring you have checked to include all sub-folders.
Then once exported you would import into the shared mailbox you have set up. You may follow the steps given into this informative blog :
https://ctrlexchange.wordpress.com/2013/07/26/convert-public-folder-to-resourceshared-mailbox/
Moreover, if you wish to accomplish this task automatically, you may also consider on this automated application (http://www.exchangemailboxtopst.org/) that could be a good approach to get this job done
in more easier and secure way.