Hmm, well. It is possible!! History states that monsters are capable of absorbing a human's soul, if only because unlike our own, a human's soul will persist after death!! The result would be, and I quote, "a horrible beast with unfathomable power." It all sounds ghastly to me, I'd rather not imagine the result, or even the circumstance which would bring it about!

[He's being a bit of a hypocrite, because hadn't he wanted a human soul specifically to leave the Underground? What, then, would he have had to do with it to cross the barrier? But no, Mettaton prefers to ignore that detail. Anyway, this human is lucky he has a good memory for all this. Mettaton really doesn't put much effort into memorizing things he doesn't care for. However, since he's a robot, he has the ability to store such information for later reference.]

Why are you so interested? There's no reason to think a human would cannibalize the soul of it's own kind!! Lending thought to such an awful thing...it's better to joke about a cake with human soul inside it than think too hard!!