From my understanding in setting Smoothwall RADIUS setup:
1) Wireless Controller
- use the RADIUS auth against your RADIUS server(ususally domain controller)
- set the Smoothwall as your RADIUS accounting
2) On your Smoothwall
- wireless controller set as a client and pre-shared keys are installed to the wireless controllers
- RADIUS accounting should be set to NPS
If I'm not mistaken this should let you achieve what you're looking for.