Use username for identity federation for SSFF and SAML2.0

Dear experts

In the SAP documentation it is stated that SAML 2.0 Protocol only considers the user ID and uses this ID for identity federation.

SAML Successfactor

However,in my instance the user IDs and user names differ from each other.

We can solve this using in the field Alias the userId, however we not want this because to we not use the Alias in the t.code SU01

My question is:

How can I modify this and that the SAML 2.0 Protocol can consider the username or How/Where can add other attribute as username?

And in the t.code SAML2 use in the field Assertion Attribute Name = username

Regards