I have "snuck" feedstore chicks under broody hens many times, because like you i wanted breeds i didnt have. Over time i learned which hens would gladly accept any and all chicks, & which hens would reject or even outright kill chicks they recognized as not their own. BUT, i also finally discovered a 100% fool-proof way to give feedstore chicks to those same broodys that had previously rejected chicks they hadnt themselves hatched. My key to 100% sucess is this. After the broody has hatched all her own chicks, but BEfore she ever takes her own chicks out of the nest box for the first time, put the chicks u want her to adopt in the nestbox with her and her own hatched chicks. Do this at night in the dark, when hen and all chicks will be quiet & still. The adoptee chicks will have all night to get cozy and comfy under mama hen & snuggle with her hatched chicks. The next morn, when the broody hen takes the chicks out and sees her new family for the first time, she will accept All the chicks as hers. 100% sucess rate even with a hen who previously killed introduced chicks. I agree u will likely need to raise your current chicks separately in a brooder, since your broody has seen and rejected them. But NEXT time, i believe she will accept adopted chicks if u take the above action. The whole key is dont allow her to see her own hatched chicks until after u have introduced the ones u want her to adopt.