Hi Ruber,
I have worked on this type of requirement. To let the users login to a webpage, you may find the script JUNIPER_NETSCREEN_WEB useful. This script lets the users login to a webform to manage the Juinper Endpoint. You may this script with the default PIM installation . It is located at C:\jboss-4.2.3.GA\server\default\deploy\IdentityMinder.ear\config\sso_scripts.
You need to change the HTML elements to input the username and the password. If you are unable to achieve this, please post the HTML source of the webpage. I'll edit the script and update.
Thank You