This can be achieved using a Tun/Tap device as suggested by psv141, and manipulating the /Library/Preferences/SystemConfiguration/ist file to add a NetworkService based on either a tun or tap interface. Unfortunately, no interface is resolving DNS names if none of them are inactive. By configuring an interface to use the virtual DNS server, I thought I could test some DNS scenario's. Those virtual machines can be reached by their IP address, but not by their DNS name, even if I run a DNS server in one of those virtual machines.

Even then, it makes sense to have networking capabilities when running servers in a VMWare installation. This scenario is needed when working in a completely disconnected situation. You can create a new interface in the networking panel, based on an existing interface, but it will not act as a real fully functional interface (if the original interface is inactive, then the derived one is also inactive). I know that you can make a virtual network interface in Windows (see here), and in Linux it is also pretty easy with ip-aliases, but does something similar exist for Mac OS X? I've been looking for loopback adapters, virtual interfaces and couldn't find a good solution.

Mac ethernet cards