Wireless ap client list from within pfsense?
is it possible to monitor and manage wireless clients from within pfsense using an external wireless AP?
Monitor and manage in what fashion?
You can use bandwidthd to monitor bandwidth utilization by IP.
You can keep an eye on your DHCP leases, too from Status–DHCP Leases.
Other than DHCP leases, etc, you are looking for functionality typically found in a wireless controller.