Really, they've got different designs, but you could seriously take the ghosts from Find Mii and shove them in Dark Moon and I don't think anyone would think them too out of place:
Compare them (ghosts from StreetPass Quest):
(pictures from Nintendo Wiki)
To the ghosts from Luigi's Mansion Dark Moon:
Really, make 'em a bit more glow in the dark/neon coloured in appearance and you could probably use them as Luigi's Mansion 2 enemies pretty easily.
What do you think?
Compare them (ghosts from StreetPass Quest):


(pictures from Nintendo Wiki)
To the ghosts from Luigi's Mansion Dark Moon:

Really, make 'em a bit more glow in the dark/neon coloured in appearance and you could probably use them as Luigi's Mansion 2 enemies pretty easily.
What do you think?