From flipping through FC1, I noticed that the dybbuk (1 of only 2 loumara) don't really use the full possession rules. They just take over a corpse. So, to simulate the possession rules, let's steal from malevolence for the spirit-ghoul to become a "rider" and grant the victim a second save to resist transformation (successful save means immune to transformation for 24 hours). So we'd essentially be writing a statblock for the incorporeal fiend and a small template for the transformed victim.
How's that work?