A Bafang to LIGHTEST adapter seems plausible.
There have been such mods between Bosch and bafang in the past.
Edit: I have no idea how you would attach the spindle/BB though.
Given M600 > Lightest conversion, I'd be curious how much weight you actually save, though. Considering the M600 is essentially an 'all in one' package. But when you use the 'Lightest', you still need to add a bottom bracket, bearings, axle, wiring, controller, etc. Seems like that is going to be quite a bit of weight in addition to the 'light' drive unit.
Just looking at their image, it seems you need - obviously the drive unit, #1, but then you also need parts 6, 7, 8, 9, 15, 16, 17, 18 and 19 just to make the Lightest equivalent to the M600. Then you'd also need some Lightest to M600 mounting brackets and additional bolts. I could see all that being a few extra kilos - and suddenly the Lightest is no longer the lightest.
