Both .mg2 and .ls3 files come from Band-in-a-Box software, but are used for different purpose so a direct mg2 to ls3 conversion doesn't seem very likely. But it might be perhaps possible to create a new style based on a song, so in this sense this is perhaps doable. But we have not found any information about this and you might be better of contacting BaB support.
Updated: July 18, 2023