add a wireless router by connecting it from the WAN port on the router to the switch
I see the confusion here - the WiFi device is one of these that is also a router - has a few LAN ports and a WAN port.
Normally you just ignore the fact that it has a WAN port - put tape over it. Plug one of the LAN ports into your LAN switch. Switch off DHCP on the "WiFi router". Just have it offering WiFi, the DHCP will come from pfSense, through the LAN switch, through the WiFi device and delivered to WiFi clients.
You could, as I think you were meaning, connect the WiFi-router WAN port onto the LAN switch. The WAN of the WiFi-router will get an IP address handed out by DHCP on the pfSense LAN. If you static map this address in pfSense DHCP, then you know that all your WiFi clients will be NATed behind the WiFi router. I guess that has some advantages - WiFi devices can't set themselves a static pfSense LAN IP, your pfSense LAN firewall rules can be sure about the source IP address of all traffic from the WiFi. That might help with a bit of control of WiFi "guest" devices.