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.