SAML vs Kerberos username formats

If we have SAML authentication used in some location and Kerberos used in others, how can we add multiple username aliases on a single user ID? Our Kerberos realm (AD forest) name does not match the SAML username suffix (user UPN).

For example:
Kerberos: usera@oldcompanyname.com
SAML usera@newcompanyname.com