Little Navmap currently supports what Microsoft Flight Simulator offers to import as a flight plan. .lnmpln is the file type of Little NavMap and I think it’s unlikely this would be adopted by Microsoft; rather Microsoft would set a standard for the flight plan format for the sim.
If the flight save / flight plan format would be further developed, the author of Little Navmap could update the software to support the new file type format.
At the moment the basic save / load feature is in an incomplete state.