It says based on Fereldin lore, with in the codex. That's basically as rumor of a rumor.Any thing like that is just speculation like spirits being the children of the maker.
Fair enough.
What's interesting in the Codex entry is the line, 'able to spread a curse to those they bite that would drive them mad with unthinking fury. When in this enraged state, a human host can likewise become possessed and be transformed into a feral, wolf-like beast.'
A rabies like disease that induces rage and acts as an open door for other rage demons would explain how it spreads, rather than the entirety of the werewolf disease being in the bite itself.
I'm still not clear on why the Lady had to be forced into Witherfang to create the Brecilian variant. Did the Lady provide the magical energy, and is irrelevant otherwise? Did it have to be her and Witherfang together? It seems like Zathrian had a specific ironic fate for the humans in mind, which brings up questions of his previous experience with curses and werewolves.
Of course, that's all speculation. 
Whether Dryden was possessed before or after death, Anders proves that spirits can possess Grey Wardens. If the Blackmarsh werewolves are possessed by rage demons, then it's proven it's possible for the host to be tainted while the spirit is still possessing it. But darkspawn could be different by way of being soulless husks subservient to the Song. Anders and Justice can fall prey to Corypheus in Legacy.
Taking this further, can Archdemons be possessed?
A Pride demon possessing Urthemiel for the thrill of the hunt?