Yeah, you can build up default to add clustering.
Suggest you start by copying default and making a "cluster" config. This way you'll still have the original clean default to start with.
Then copy jgroups.jar and jboss-cache.jar from all/lib to cluster/lib. If it's there in all/lib and not in cluster/lib also bring over jboss-serialization.jar.
Then see the http://wiki.jboss.org/wiki/Wiki.jsp?page=JBossASTuningSliming wiki page; near the bottom there is a " If you do not use clustering" section. That shows how to remove clustering services from "all". Use that as a guide to tell you what you need to bring over from "all" into your "cluster" config.
Can't guarantee you won't hit some bumps, but that should get you far enough down the road.
Thanks for the guidance on this. Skimmed over the tuning guide and your suggestion makes sense. Thanks for the help!
Forgot; you'll also need jbossha.jar from all/lib.