Darryl,
You can do the same function but you can not create the users internally and point their password authentication to external servers.
ACS 5.x has a new feature which is called identity store sequences...for example.
UserA has an account that is stored locally on the ACS internal database and UserB has an account on the radius token server.
You can configure a identity store sequence which can either look in the Radius token server first and if the user is not found then will look for the user in the interal database. Then you can tie this sequence into your access policy.
Let me know if that helps,
Tarik