Is SAML Authentication possible for AWS SSO?

IDp Source: AWS SSO, Accessing Appian app. as external application thru AWS SSO SAML authentication.

Could you pls suggest how do we import / upload  the signing certificate in AWS SSO? I could not find any option to upload the certificate in the "Manage Certificate" section.  
Is this something that we can't do in AWS SSO whereas this is allowed in Azure AD SSO.

The AWS SSO certificate which we can download does not contain the private key. My SAML Appian SP application expects to configure SAML settings with private and certificate data in the combined . pem file.

In AWS SSO, I'm unable to upload / import the certificate file that I've configured in Appian application. Does it means AWS SSO SAML authentication not compatible to integrate with Appian?

We've managed to successfully configure and test this with Azure AD SSO. I'm trying for alternative approaches of integrating AWS SSO Idps with Appian app.

Thanks.