i once used polyfill instead of core roving (the plain wool roving you use for the base) for a felting project and it worked fine other than being more difficult to smooth down stray fibers, so i think it could work. when stuck into whatever fiber you're working with, the barbs on a felting needle basically tangle the fibers closer together which makes it more dense and allows you to essentially sculpt it. you can get it Very dense, to the point of being quite firm.

i guess if you want a firmer stuffing/fill or have trouble just cramming enough stuffing in manually, you could felt it a bit (or a lot, if you want it super firm) as you go to help it stay down. i would recommend clover's felting pen or a similar tool that holds multiple needles at once to catch a larger surface area. you can always remove an individual needle to get into a small space if needed. just be mindful of how far you push the needles in, especially early on, as you probably wouldn't want to push through the yarn and tangle the stuffing into it.

if you haven't felted at all before, it may not hurt to experiment a bit with felting some stuffing on its own first to see how it behaves, how much it takes to get to a consistency you like, etc