It looks like my USB wireless network adapter is appearing as ugen0, but
says that ugen0 is not an interface. Interestingly, it also identifies the chipset as GlobespanVirata Cohiba 3887 rev0. It turns out that GlobespanVirata has merged with Conexant, and on the Conexant drivers page there is a link for Linux drivers for this product to
www.linuxant.com who have developed the product DriverLoader for Linux which works just like ndiswrapper, but they don't have any equivalent for BSD. But I haven't found a solution yet.