Not if you enable anonymous access. See the "Managing Anonymous Access Settings" section of the Administrator's Guide for the full info on how to do this.

SharePoint user accounts  are required  since customization is part of
the requirements. Can I create SharePoint users using any unique
userid instead of windows domain  accounts?

I am having this problem because an internal SSO authentication has
to be used. If I can register my SSO userid as SharePoint user, then
it will be very easy to support customization and personalization.

SharePoint depends on the users being authenticated by either Windows NT
or Active Directory. SharePoint only does authorization, not
authentication. The SSO is a one-way process, meaning that the user  is
authenticated by NT or AD, but not by an application with is then ported
to SPS.