This is how you add users to an administrative role, such as Administrator role or iscadmin role (taken from http://pic.dhe.ibm.com/infocenter/wasinfo/v7r0/topic/com.ibm.websphere.express.doc/info/exp/ae/rxml_7libsecurity.html):
Syntax
AdminAuthorizations.mapUsersToAdminRole(authGroupName, adminRole, userIDs)
Example usage
AdminAuthorizations.mapUsersToAdminRole("myAuthGroup", "administrator", "user01 user02 user03")
Also have a look at the links in the InfoCenter as they will help you with all the related tasks as this will help you with all related wsadmin commands.