I just did this last week with one week old chicks and it worked perfectly! I had a hen hatch 3 babies on her own but got some one week old chicks to slip under my blue laced red Wyandotte. She was thrilled. The two moms are now co-parenting
The strategy is... to wait till night time when shes asleep and slip the little-uns under one at a time... each one will worm its way under for warmth... She will wake up in the morning with Babies... I got babies... LOL... There is a chance they wont be accepted always the chance but this is the way to give the best odds.
So slip em under at night and keep an eye out in the morning
deb