Ah, maybe I'm lucky with the sim I use (http://heli-x.net) , If I export to an .obj file and fiddle with parameters in an xml file (copied from an existing model and adapted to match rotor positions, weight etc.) I can make it fly.
In fact I just spent some time updating my model (added LED strips and refined the batbone shape):
I've tried to look at Phoenix RC, but that is impossible, they use their own (binary) model format.
I've heard that if you request a model they are willing to look into it.
Maybe it helps if you already have it in a popular export format (.obj, .stl, etc)
FMS and desk pilot RC also have a different format, not binary, but also not easily converted as far as I can see.
The others (RealFlight / aerofly) I cannot try, because I don not own them....