Yes, you can add a wireless nic and either use it as accesspoint mode, infrastructure or adhoc mode. What you want is accesspoint mode and maybe just bridge that one to the lan interface though you also can have a seperate subnet on it. Depends on what you want to do with it.
Only make sure the wlan-nic is supported by freebsd. I would simply go with an atheros chipset based card. Always worked fine for me.