How do I route stereo, 5.1 and 7.1 in 3rd order Ambisonics and preserve the original image?

Route a stereo track into a 5.1 bus, Nuendo “knows” the stereo routing (front L and R) and the audio will pass through with no alterations.
Do the same with 5.1 to 7.1.

Route any of those to a 3rd order ambisonics bus and you get… nothing of the sort.
How can we route a 2.0, 5.1 or 7.1 directly into a 3rd order bus and retain the original mix imaging?

Use FX Send and send to an audio output bus, then downmix or up mix using appropriate Steinberg plugins on that bus. You can even export all the busses at the same time using multi channel export