In any case is it likely or possible that even the least of the Maiar would become Orcs? Yes: both outside Arda and in it, before the fall of Utumno. Melkor had corrupted many spirits - some great, as Sauron, or less so, as Balrogs. The least could have been primitive (and much more powerful and perilous) Orcs; but by practising when embodied procreation they would (cf. Melian) [become] more and more earthbound, unable to return to spirit-state (even demon-form), until released by death (killing), and they would dwindle in force. When released they would, of course, like Sauron, be 'damned': i.e. reduced to impotence, infinitely recessive: still hating but unable more and more to make it effective physically (or would not a very dwindled dead Orc-state be a poltergeist?).
But again - would Eru provide fëar for such creatures? For the Eagles etc. perhaps. But not for Orcs.
Tolkien, J. R. R. The History of Middle-Earth X: Morgoth’s Ring. Ed. Christopher Tolkien. (London: HarperCollins, 2002.) 410. (Myths Transformed, Text VIII “Orcs"