Add users from SSO server database to Drupal
IMHO, integration of different system through form POSting is at the hack level. To let external system inject data into Drupal, the Services module let you easily build HTTP REST API in Drupal. It comes out of the box with a user API that allow external system to easily create new Drupal users.

Off course, you can create your own page to receive POSTed data from a form build outside Drupal. You simply have to declare it through a hook_menu() implementation. For access control to the page, you can either use the 'access arguments' to restrict access with Drupal permission system. you can also use your own 'access callback' to implement your own access control logic (ie. for instance, using a secret token based access control).