I don’t know how SlinkyMate has done it, and would be very interested to hear that as well.
But I’ve had some partial success by creating a “package” of the FSX aircraft folder. Or at least something that MS FS will recognize. Basically so it ends up in a file structure like:
[MSFS dir]\Packages\Community
klasbj-goose\
manifest.json
layout.json
SimObjects\Airplanes\Grumman_Goose_G21A\
manifest.json and layout.json seems to be what makes it a “package”, containing some metadata and a list of all other files that should be read by the simulator respectively.
It seems to load the graphical model, flight model and main textures fine. Though I’m having quite a few issues with gauges not working (at all, it seems). It also seems to have trouble with (at least some) specular map textures.
I tried creating the packages using the SDK at first, but it didn’t seem to want to include all the files. In the end I created a program that copies a directory and generates layout.json since I didn’t want to do that manually. (Available here: https://github.com/klasbj/planeconverter, if anyone’s interested.)
Edit: To preempt some common questions:
You need to .NET Core 3.1 SDK to build the program. (It can also be installed as part of VS2019.)
While the program may have been somewhat ambitously named, it doesn’t really convert anything. It simply packages FSX structured airplanes so they can be loaded into MSFS as is. Most aircraft have some issues, e.g. it appears that only 3D gauges work, since they don’t use a generated texture.
It’s currently unlikely that the program will see further development as a converter, as I don’t see how it could resolve any of the issues I’ve seen so far.