Yes:
- Setup a VLAN
- Give it an IP address on its interface. That will be your client's default gateway.
- Setup a DHCP scope for that guest range with the controller's ip address on that VLAN as the default gateway
- On the IP interface of that VLAN make sure "ip nat inside" or "source nat" is enabled
- Change the Virtual AP VLAN for your guest network to the one you just setup.