I don't think it'd be an issue for them at all but take that with a grain of salt as I haven't raised ducks. Rabbit pellets are mostly grass and alfalfa around here. My
chickens will even happily eat the other rabbit pellets (
poop) for the undigested feed value.
As for duck feed, I don't know where you are, but call the feed mills around you. Here I can get non-GMO feed for not much more than conventional feed or organic for a 40-50% premium. I do know that you can get feed shipped, though that's expensive. There's H&H in Texas, I think, and sometimes you can get it shipped to a
local feed store for cheaper. It all depends on how much you're willing to pay for the feed you want, but hopefully a little (or a lot) calling will get you sorted.
You
should also post where you are as someone local might be able to point you in the right direction. BackyardChickens.com would also be a good place to post for help finding a good feed in your area.
Good luck.