This question seems to suggest a lack of understanding of what FireSheep actually does.
Anything is possible with enough effort / resources.
FireSheep is a simple UI for HTTP packet sniffing / packet analysis. If you have secure access to a secured wireless network, then watching network traffic within that network is probably simple.
In short, FireSheep doesn't have the ability to inspect traffic in:
- any non-web traffic (including HTTPS/SSL connections)
- VPN connections (an encrypted tunnel of TCP/IP traffic)
- networks which you don't have access to (which includes secured wireless networks)
Was the question meant to ask if it's possible to analyze and inspect hardware that is logged into the WiFi network? That seems like asking "is it possible to make a flying car?" when airplanes already exist and serve that purpose. In order to see any traffic (like what FireSheep does), you need access to the unencrypted network traffic.
If the question was meant to ask if it's possible to use FireSheep (or a modified version of FireSheep) to inspect the same sites, only on a secured wireless network you don't have authorization into… then your task is not to build a tool like FireSheep, but rather to break into a secure network (which means once you're into the network, you can simply use FireSheep).