if I correctly understand that, so far, it is not possible to import OpenSim [1] models in a similar way to URDF and MJCF importers, then how may I implement a new importer for OpenSim models?
Your best bet would be to look at Isaac Sim for this kind of work. Isaac Sim is designed for extensibility, and there are example connectors you can use to do something like this. See here for more info: Connect Sample — Omniverse Connect documentation