It's a possibility, though it's never been confirmed or denied. And as other users have noted, it would be difficult to persuade someone that Olaf was Numinex in human form if people had never heard of such things.
I would guess not, though. Within the metaphysics of TES, there's a very clear distinction between mortal and not-mortal. Dragons fall into the latter category, which is why "Dragonrend" works: it forces them to contradict their not-mortal nature by understanding mortality. Because of this barrier, it seems unlikely that a dragon could take on mortal form, in the same way that they can't use the Dragonrend Shout.
Technically, though, the Dragonborn could be considered a dragon in humanoid form (although s/he was born that way), and s/he doesn't appear to be in constant spiritual agony.
It's a possibility, with equal weight, I would say, on either side of the yay/nay divide. On /r/teslore, we might tell you that "they can if you want them to". The open world nature of the games means that all our experiences and headcanons are welcome.