It's certainly possible. I would question, though, whether the Fallen Lords' personalities (especially Myrdred) really change abruptly enough to justify saying they have a whole new spirit inhabiting their bodies. And also, you'd think they'd have a bigger impact on Ghôl religion--which they might, who knows, but at least we've never seen the Ghôls actually worshipping a Fallen Lord.
Beyond that, it's mentioned that Balor gave Myrdred his new title of "The Deceiver" after bending him to his will...as opposed to a Dark God possessing Myrdred and then announcing that to be his name. And the Fallen still seem to refer to each other by their old names, especially when they're arguing...they certainly don't act like their memories of the old days aren't really theirs.
All circumstantial, of course...I'm just saying it seems (to me) to fit a little better with the Black Company brainwashing model than with actual possession by outside beings.
