I have a bespoke web application accesed through forms
authentication where USER role and security is stored in SQL. I want
to give access to user from this application to WSS a specific team
site based on his security. How Can I pass user credentials to WSS
without WSS prompting for user name and password like a single sign
on.The link between web application and WSS has to be seamless.

There is no active directory because some users can not be added to
domain.

I know the reverse works for single sign on from WSS to access custom
application.