no, what you can do is to subclass org.jboss.nukes.core.modules.CoreModule and redefine the user based methods

public UserEJBLocal currentUser() {...}
public void userLogOut() {...}
public boolean userLoggedIn() {...}
public UserEJBLocal findUserById() {...}
public UserEJBLocal findUserByName() {...}

you have so to implement the interface UserEJBLocal which is easy to do.

if you want also override user creation you have to redefine public void finishnewuser(Page page) {...} in org.jboss.nukes.core.user.UserModule

julien