Hey, I'm too lazy to type out everything you need to know, but what you're looking for is in http://docs.jboss.com/jbossas/admindevel326/html/ch11.chapter.html
This is not a recent copy. If you get a username and password for jboss.com, you can access the latest docs in the Documentation section. The section in the AS Manual that you're looking for is Chapter 11, just like the link I posted.
It starts with setting the primary key class to java.lang Object, removing the primary key field, and then using the jbosscmp-jdbc.xml or standardjbosscmp-jdbc.xml file to configure how CMP should handle the unspecified PKeys.