Hi Jiri,
the ABAP application code doesn’t have an access to the assertion attributes. This is because it is assumed that it should not depend on the authentication procedure.
There is the option to map saml 2.0 attributes for transient users but here usually only groups of external users (for example members of the purchasing organization of a third party) are mapped to one technical user.
http://help.sap.com/saphelp_nw73/helpdata/en/4a/b619153fec6d83e10000000a42189c/frameset.htm
There is also be the option to create users that have been authenticated using SAML 2.0 on the fly
http://help.sap.com/saphelp_nw70ehp2/helpdata/en/46/631b92250b4fc1855686b4ce0f2f33/content.htm
Hope this helps.
Best regards,
Andre