Hi @dcarrion
There is a Normalised RADIUS attribute in ISE called "Normalised Radius SSID" that you should use for this.
Behind the scenes it looks at the RADIUS attribute called Called-Station-ID, which normally contains the SSID in ASCII format. I say "normally contains" because it's up to the WLC configuration whether or not the SSID is contained there. By default I think it's the AP's Ethernet MAC address. But you need to tell the WLC to include the SSID name into the Called-Station-Id. Then you can use that in your ISE policies.
cheers