Share internet connection in OSX through existing wireless network?

Question

It's easy to share your Internet with AirPort by checking "Internet Sharing" in the system preferences, but this option asumes you want to create a new wireless network. Is there an easy way to configure Internet sharing if you want to use an already existing network?

I want my laptop to use the existing wireless AP and share its internet to other clients within this network. Tried manually running natd -interface ppp0 without luck

Answer 1

Got it working

For anyone interested, these were the steps:

1. sysctl -w net.inet.ip.forwarding=1
2. natd -interface ppp0 -dynamic
3. ipfw add divert natd all from any to any via ppp0