The point I am trying to make is that IP cameras don't need another encoder, the encoding process is built-in. If Wowza can connect to the IP camera it can re-stream from it. You can have an IP camera on a private IP camera, and restream it to public internet from a Wowza server, as long as the Wowza server can connect to the IP camera, and clients can connect to the Wowza server from the internet.
I would suggest testing Apple's QuickTime Broadcaster to see if it can push out from your private network to your remote Wowza Server. I haven't tested this function, but if you have a Mac, it's free to test and the setup guide is posted in the Live Encoders section of the forums (you'll notice in that guide that no where do you give the Wowza Server the IP of the encoder).