It depends-- some chicks seem to start running day two onward and some need a few extra days. If it is up and walking, or the hen stays relatively close to the chicks and they don't go adventuring far enough that this new one would have a hard time keeping up you're probably fine to sneak the chick in.
Obviously night time introduction or plan on spending a lot of time supervising if you do a daytime introduction, being ready to swoop in.
I did read an interesting write up about using artificial vanilla extract, wiping the chick with it and covering your hand so when you place the chick underneath the hen at night you make everything you touch, hen and chicks, smell like fake vanilla and the hen is less likely to abandon it for not smelling like her like the chicks who have been with her longer would.
But I have no firsthand experience with that.
In the future, keep some cornstarch by your incubator. If you find yourself needing to help and there is some blood, pinch some cornstarch on to the blood to stop the bleeding. Styptic powder is also useful and does the same thing but most have cornstarch. I have used it several times to good effect.
And if there are walking issues, hobbling and cup therapy only takes a few days. Hopefully you don't have any bent toes because taping their feet is annoying (but also takes less than a week and do recommend it. I gave up on a silkie I have and feel bad every time I see him, though it doesn't appear to slow him down at all. I worry it causes him pain.)
Obviously night time introduction or plan on spending a lot of time supervising if you do a daytime introduction, being ready to swoop in.
I did read an interesting write up about using artificial vanilla extract, wiping the chick with it and covering your hand so when you place the chick underneath the hen at night you make everything you touch, hen and chicks, smell like fake vanilla and the hen is less likely to abandon it for not smelling like her like the chicks who have been with her longer would.
But I have no firsthand experience with that.
In the future, keep some cornstarch by your incubator. If you find yourself needing to help and there is some blood, pinch some cornstarch on to the blood to stop the bleeding. Styptic powder is also useful and does the same thing but most have cornstarch. I have used it several times to good effect.
And if there are walking issues, hobbling and cup therapy only takes a few days. Hopefully you don't have any bent toes because taping their feet is annoying (but also takes less than a week and do recommend it. I gave up on a silkie I have and feel bad every time I see him, though it doesn't appear to slow him down at all. I worry it causes him pain.)
and legs seem to be in the right position beneath her although she is mostly sitting on her hocks and is pretty unbalanced when she straightens out. She is moving around fairly well and has had a drink of some electrolyte, feeling cautiously optimistic 
. 
