ATG is a proprietery framework and it is using so many proprietry features like Droplets, FormHandler etc.
If your war file contains of plain jsp files and you want to use it with ATG engine, it is not possible. You need to rewrite your JSPs to leverage ATG features.
You cannot migrate with minimal code effort, migrating a normal JSP/Servelets application to ATG means you are going to rewrite everything. You can copy-paste some of the business logic code but that doesnt means that you are re-using.