The Spatial Mic Converter is a 8 in / 12 out plug-in. Even though it’s using Ambisonics, the I/O configuration is not. You can work around this limitation by using Plogue Bidule plug-in (or similar) to host the Spatial Mic Converter within Nuendo.
You could also use Reaper to convert the files from A to B-Format before bringing them in Nuendo.
Well, as it appears in Bidule and Reaper, the Spatial Mic Converter is only a 8 in / 12 out, no other I/O options. I’m not saying that it can not output 2nd order signal, but it does so on a 12-channel output bus.