I think you can use a rule to modify the value of your user_id in the SAML assertion. The downside is that rules are deprecated and will cease to be available on november, 2024. Using a “Change SAML configuration” is the best option at the time: either you map another value on the nameId, or you transform it with a string function in the rule. Doing this implies you migrate the rule into an Action sooner or later. Is this of any help ?