I recall starting to add grower into the starter during their third week. I ground up the pellets at first, and watched carefully to make sure they could handle them. For a while I also had some ground up oatmeal (not instant - the rolled oats) because the feed I could get was a little high in protein.
Each day I would add a little more grower and a little less starter crumbles. By the end of about ten days they were eating the grower feed without needing it ground up.
I kept my ducklings on start n grow crumbles till 8 weeks old roughly then switched to Purina Flock Raiser until they began laying then I went to Purina Layena. I switch baack to Flock Raiser when they are not laying.