Is there an event that can be captured during the Portal SSO login process on the successful SSO login?

I know I can create a Creator Portal user/adjust a user's permissions via Deluge but, is there an event that fires on successful SSO login that will allow me to associate a workflow or function to it?

We have a registration toolset in Creator that is configured for SSO.  SSO functions as expected.  When a user tried to connmect and does not have a portan user account, they get a generic error that they dont have a user account which is confusing since they just logged in via SSO (so, they have an account there). 

Ideally, I would like to trap the successful SSO login event and execute a script to add them as a portal user. Is this something that is possible?

Note - I know I can pre-load portal users and avoid this as an issue.  However, we have 25k+ customers that could register.  Last year, we had 2500 complete this registration.  I am having a hard time justifyting the cost for that many portal user licenses when a fraction of the total will actually leverage this application.