Well, login in using security assist is done by looking up the username and password that is entered, and storing the ID for the user in a session variable
It may be that the osCommerce system works in a similar fashion, but it may work differantly.
since the login system is not created by security assist, there is no way to know what that session variable is called, if indeed it works in the same fashion.
I'm sure that if osCommerce has a login system, it can also be used to restrict access to pages.
security assist is intended to be used with a login / registration system created by security assist, not one created by a third party