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.
Co-author of SCMAD Exam Guide, Author of JMADPlus SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0
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.
Java Server Faces Portlet Tutorial - Code and CBT for JSF Portlet Development