With VMware Fusion 5 Professional there is a Virtual Network Editor in VMware Fusion's Preferences. So between that and the Virtual Machine's Network Adapter Settings GUI the same should have been able to accomplish without manually editing the networking file. If you don't have VMware Fusion 5 Professional then manually editing is the other native option however there is a third party tool out there that can do it also although I'd opt for manual editing if I didn't have VMware Fusion 5 Professional.
Really? If I go into the preferences it will allow me to adjust new custom networks, but all of the default supplied networks are unmodifyable. So if I wanted to remove NAT and Host-only, I could not (via the GUI).