It's unusual for demons to possess non-mages. We see it in DA2, where the Xebenkeck cultists are putting demons into templars, but it seems to involve some heavy-duty magical assistance. Sophia Dryden... I'm not sure if she was possessed before or after her death. I always figured her for a revenant.
If your Orlesian Warden is a mage, there's definitely nothing in the Joining that prevents possession. If your Warden is not a mage, be prepared to explain how the possession occurred. There's lots of mumbo-jumbo you could throw at it: Veil tear, something happened while Fade walking (as happens to non-mages under Sloth Demon influence in DAO and via Marethari's spell in DA2), tortured by cultists like the templars in DA2.