This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Maybe, Liferay could provide better solution for you to migrate from existing frameworks to portals and portlets.
I heard that the way Liferay supports migration of portlets is that we can migrate JSR-168-compliant portlets to Liferay, but we can migrate Liferay portlets to other JSR-168-compliant portals. I don't have any experience in migrating portlets across different platforms like this. But this is what I heard from the Liferay communities.
If you're using JSF, you might find this JSF portlet tutorial helpful. It demonstrates how to take a Java Server Faces application and use the apache JSF bridge from the apache bridges project to allow the JSF app to be deployed as a portlet.