mic surround techniques to Ambisonics

[lamacchiacosta]

Hello,
I am trying to make my head around how to convert/decode stereo, multichannel surround techniques to Ambisonics, (pretty much the way round of most Ambisonics decoders). I would like to adapt my old recordings to my new spherical projects. Let's say I have a recording made with a stereo technique for the front and a stereo technique for the back. I know the distance between the mics: how can make that into ambisonics stream (AmbiX)?
So far I found some plugins that could be in the right direction but they don't seem to do what I need:

Waves B360 Ambisonics Encoder - looks like more a mix-to-ambisonics tool
Harpex-X - It seems the only technique it can manage is DoubleMS (which I never used).
SPARTA Array2SH - to me is not very clear what it does.

Thanks.