Hm... perhaps both? Not only can you be born a Fallen Angel but you can also become one? Or maybe those born Fallen Angels and those who are corrupted into them are two different, separate species? To be perfectly honest, I haven't thought it all the way through... I have a slight problem of trying to go a "
All Myths Are True" route and bring all interpretations in, so it becomes a bit of a mess...
Also, I've been doing some thinking on Belial and Mammon, and I think I have a few ideas that I should run past you guys.