How to provide different credentials to the service being called?

[BlueBerkSebi]

Hi,
So basically i want to execute a SAP rest request, but the SAP server is protected by SPNEGO.
When using basically the example code snippet, i get a successfull authentication with Kerberos,
but the SAP is unhappy, as it can't find valid credentials in teh Authorisation header.
So when i have different credentials for SPNEGO and SAP, how do i add/switch my authentication after i logged into SPNEGO?