How to limit an app's access only to specific networks?
You can use rules like this in your custom scripts:

$IPTABLES -A "droidwall-wifi" -m owner --uid-owner 12345 --destination "192.168.1.0/24" -j RETURN


Instead of 12345, you need to specify the UID assigned to the app; you can find that UID in the list of apps in DroidWall.

Remember that UIDs are assigned dynamically, and if the app is removed, the UID previously assigned to it may be assigned to a completely different app installed later.

There are some enhancement requests for DroidWall (#275, #253) requesting to add such feature, but developers did not respond to them yet.