Some of my questions were not accepted well; I'll try my luck with Meta first.
To formulate the topic/subject: given an auth API for user/group management: create users with passwords, add them to groups, like this.
On the other end there is KeyCloak which supports SAML.
My question is at best how I can formulate it: while I think whether it could be possible to implement some intermediate software/KeyCloak plugin to integrate with that auth API, I think the answer would be yes. (like, dynamic users during a session, simulate user login and pass a cookie, these are details)
However I am not sure how to validate this idea conceptually with some high chance of probability - is it possible without starting an actual trial and error project? From reading the docs of KeyCloak custom extensions I am not sure.
Not sure how better formulate this architecturing/design question and where to start in this specific SAML/IAM context.