There are several parts to the problem:
1.) You need a model, strictly in the 3D sense. There are some in the Internet to buy, but they have different licenses and you may want to ask the authors about that, but it comes down usually to CC agreements or sometimes, someone says, fine for the flightsim, but don't give away the model alone, and again other have built it for other games for free, but it became part of bigger and licenses packages (because someone took their GPL work and places it in his work and "forgot" to respect the original license), and say, okay, why not FG too? So, asking authors can give you the most surprising results.
The other way is, you find someone here who can build the thing, for example in Blender. I know, IAHM-COL has used Blender also to convert other formats, because in the end, you need it in .ac for FlightGear. Some few people can do that, alas, I was never good enough in 3d
2.) once you have a model, it can be used to build the helicopter for FG. Animations, engine simulation, FDM, all that stuff. Currently, almost all helicopters run on Yasim. So you can try to find a helicopter not too different as some kind of inspiration. "Not too different" as in maybe as a guideline for a Sioux not a zwo-main-rotor Mi-12, but something with one main rotor and a tail rotor.
3.) The whole thing build down to build something along the same file-structure with the aerodynamic information of the Sioux. Means, you basically write those files with the Sioux-data. You need for that of course the information about the Sioux, as in a lot of research about the original real world helicopter. Engines, rotor diameters, dimensions, weights, whatever you can get.
Point 2+3 scare people most, but we have a lot of guys here who know how to do it, so every new aircraft has it's own moments. The 3d part is the one that will be the problem because it seems, that is a lot of contacting people all around or find one of those rare modeler guys here who can do it. Thus, start with the hunt for a 3d modeler or someone who made already a Sioux. Once you have a 3d model, the rest can be probably done here in the community.
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