Big hens absolutely can eat chick feed. Better for them to do that than for chicks to eat layer pellets.
There are a number of commercial "All Flock" type options which are available pelletized. I used Nutrena "Naturewise" label "All Flock" in the 40# bags for a while, till my flock got too large and they started eating me out of house and home. Its "Natural" and non-GMO, but not Organic.
Compare the label in terms of crude protein, fats, fibers, calcium, etc, it looks a lot like their Nutrena Naturewise Starter/Grower but for slightly higher protein. Both have more protein, much less calcium than the Layer Feed from the same brand and label. Typically, "All Flock" has an increased amount of trace minerals for ducks and geese as well.
Plenty of people use the Purina Flock Raiser Pellets, 50# bag as well. Also not organic. They do have an Organic line, I see it stocked occasionally.
Both Nutrena and Purina are readily available in your typical
TSC, Tractor King, etc.
Nutrena has its "NatureSmart" label which is Organic, but you would end up using the Starter/Grower, they don't have an All Flock, even though the crude protein 20%, 6% fiber, and 2.5% fat are the same as the All Flock in their naturewise line - also, its a crumble, not a pellet.
I've not used it, and haven't seen it stocked locally, but "Scratch and Peck" is popular as an organic feed choice - Chewy.com, etc if its not available to you in store.
Hope that helps.
and free choice oyster shell on the side for your laying hens if you do transition to All Flock/Flock Raiser (as many of us on BYC typically recommend). The chicks, and any rooster you might add will try it - they explore the world with their beaks after all - but not needing the extra calcium, they won't continue to eat it.