I hope there might be a better solution thanks to IOSU exploit soon
IOSU will only really help with accessing the database file (assuming it wasn't already possible before - direct access to the database is how it works on 3DS and Wii)
We know the old format,
we kinda know the new format (although I personally didn't care about the facial features when I wrote that page, those were contributed by someone else and can't say much on them), BUT:
1- Obviously, "Mii 2" has more facial features. How would we convert the new ones to the old format? (choose the most similiar, of course, but see the next point)
2- Nobody that I know of has actually mapped the individual shapes to the values, and doing so is very easy but very tedious
3- The best/only "Mii 1" editor was My Avatar Editor, which has been c&d'd 3 times by Nintendo (obviously its existence implies point 2 was done, but no documentation)
4- I don't think anyone really cares, at least anyone with the willpower AND skills, to make a "Mii 2" editor
5- PkHeX probably had a great demand to add conversion from previous games to gen 6, because the official way sucks AND costs money; but to get your Miis from Wii to 3DS, you can transfer them to Vwii with homebrew (in bulk), to Wii U (in bulk and 3-click), to 3DS (manual but you only have to do it once), and hack them to match your console's identity (manual and repetitive but could be automated). Or if you only have 20 or so, just send them directly from Wii to 3DS...