SSO but the other way around

Hello everyone

I know Univention UCS is an SSO provider for SAML, but I want to use an external SSO System I already implemented like this:
UCS is the LDAP, the external SSO gets its user from the LDAP and it is the provider (SAML/OAUTH) for a few webapps.
now I would like to use the SSO to authenticate users against the Portal too.