Hey ! first I would advise you to use "SaveMii Mod", it's an updated fork from that old SaveMii app.
and then I have an idea on how you could do that : backup your NSMBU save, create a NSMBU+NSLU backup (by playing it at least once, in order for the files to be able being replaced)
then open your sd in your laptop and replace NSMBU save files over NSMBU+NSLU save files if they have the same names
else it might be hex injection or something like that