I can tell you nearly for certain that you do NOT need to write a JAAS login module for WebSphere. WebSphere does not support custom JAAS login modules for the purposes you state. Instead, what you are doing sounds more like a Custom User Registry. Read in the WebSphere Infocenter about those, and go to
www.redbooks.ibm.com and download and read the WebSphere 5.0 Security Handbook redbook. That should straighten you out as to WebSphere's capabilities for security.
Kyle