You can use Network Policy Server (NPS) as a RADIUS proxy to forward connection requests from RADIUS clients, such as VPN ...

You can use Network Policy Server (NPS) as a RADIUS proxy to forward connection requests from RADIUS clients, such as VPN servers and wireless access points, to remote RADIUS servers in remote domains. The Remote Authentication Dial-in User Service (RADIUS) protocol is an Internet Engineering Task Force (IETF) standard, and NPS is the Microsoft implementation of RADIUS. An NPS RADIUS proxy, used with other NPS-RADIUS servers, can provide authentication between two forests as well as authentication between both untrusted domains and one-way trusted domains.