Otherwise you could add an IP to the LAN interface as described here: http://doc.pfsense.org/multiple-subnets-one-interface-pfsense.pdf
Set the wirelessrouter into routing mode and have to wirelessrouter doing the DHCP.
Create a static route pointing to the IP of the wirelessrouter for the subnet behind the wireless router.

Physically this leads to such a setup:

[192.168.0.1/24]
pfSense–-----------------switch------office-clients [192.168.0.0/24]
[172.16.0.1/24]                    |
                                         |
                               [172.16.0.2/24]
                                        AP
                               [172.31.0.1/24]
                                         |
                                         |
                                private-clients
                               [172.31.0.0/24]

Logically its like this:

[192.168.0.1/24]
pfSense–---------------------- office-clients [192.168.0.0/24]
[172.16.0.1/24]               
                                     
                                        \–-- [172.16.0.2/24] router [172.31.0.1/24] –-------private-clients [172.31.0.0/24]

However this is NOT A GOOD WAY TO SET A SEPARATION BETWEEN TWO NETWORKS up.
And a second NIC is easier too ;)