absolutely Yes, you can see in the GateIn with JBoss AS 5.1.0 downloading package, there is a sample called "sample-portal" deployed by default for such things.
You could have a look on http://anonsvn.jboss.org/repos/gatein/portal/tags/3.1.0-CR01/examples/portal/ first for what beginning to do
Also, read more in the last chapters of the doc :
Here is quick walkthrough based on a real life use case. This is what we use to build the eXo Social demo portal.
The firt step is to write a PortalContainerConfig . There you will need to decide he portal container name (/portal2), the rest and jaas context names, a filename for external settings and a list of dependencies. These dependencies are the name of the wars your portal container will use to form a unified servlet context.
The second step is to define your portal war (portal1.war). Following the dependencies, this war will inherit all resources defined in the wars above.
In my example, I have 'socialdemo' declared after 'portal', so it means socialdemo.war will inherit all resources from portal.war.
Hence, you need to override only things that you want to make different.
In your case, it seems to be the user management. So you would certainly want to override files in WEB-INF/conf/organization/*
Finally, if you are using JBoss, wrapp the portal container config jar, the rest war and the portal war into one big ear and you have your portal extension.