Your question is very wide.
How and what do you need to integrate exactly ?
Integration could just be displaying a document (an HTML file for example) written with any CMS. Easy to do, just build a portlet that fetch the content form the CMS and dump it in your render method. (That's basically how the JBoss portal CMS works)
Then do you need authentication/authorization ? If so your CMS needs to be able to adapt to some form of authentication and authorization with JBoss Portal (You may not want users to log in in the portal *and* the CMS)
You may want to let people add content from the portal ? That's another part of integration...
Note: Some CMS provide some kind of JSR-168 portlet to provide *some* integration (Documentum does as far as i remember)