ADFS idP that first checks with another idP?

The authorization flow I'm trying to determine the possibility of is:

1. User goes to https://app.example.com/
2. app.example.com does not have a session for the user so it sends them to https://adfs.example.com
3. adfs.example.com does not have a session for the user
   • user does not have a session
     1. adfs.example.com acts as a SP and asks https://notadfsidp.example.com for authentication
     2. user has a session with notadfsidp so the are sent back to adfs as authenticated and in turn to app as authenticated
   • user has a session
     • user is redirected back to https://app.example.com as authenticated

Situation:

I have a "portal" that uses CAS to authenticate users. Due to circumstances beyond my control, the CAS server captures the user's credentials on login so that they can be used for SSO to various services (this is the wrench in the works). One, or more, of the services they can SSO to are backed by a local ADFS instance. Thus, the ideal situation is that when they go to one of those services the ADFS server can learn from the CAS server that they are logged in and the portal would not have to use their stored credentials to authenticate them with ADFS.

If the flow described above is not possible, is it possible to send the ADFS server some message that tells it the user authenticated elsewhere and they should get a session with the ADFS server?

Note: my knowledge of SAML is very limited and I have zero knowledge of administering an ADFS server (that's a coworker).

Edit #1: I need the authentication to be transparent. In other words, after the user has authenticated at notadfsidp any visit to adfs from app would not require the user to do anything.