Hi,
AS400 is not supported for SSO. Following is the platform matrix that give you an idea on the platform that supported (Windows, RH, Solaris for Policy server)
SAP Portal Services
If this is mandatory platform that you need to use, you can raise an certification request by create an Idea in following community. The product manager will review the request base on user vote feedback.
Hope this helps.
Regards,
Kar Meng