The easiest way to slip a chick under a hen is right after she hatches an egg. On that 1st day or two, she'll accept anything that peeps. I think the hen goes through a change in hormones when she feels an egg hatch under her body. Perhaps when the chick pips & the hen hears the peeping it triggers her incubation instinct off & her mothering instinct on. After the 1st few days, a protective hen may view an extra chick as a threat or she may adopt it. It really depends on the hen.
I never successfully added a chick to a hen who's in the setting period, but I know some people have. I think the hen needs at least a little time on eggs before adding a chick. When I tried it (during the day) a chick popped out from under the broody & she pecked at it. I believe that she felt the chicks should still be in the egg &/or keeping warm under her - not running around. The chick's actions probably confused her, so she disciplined it.
When other people have done it successfully, they add the chicks at night when the hen is sleeping & the chicks are less likely to run around & confuse the broody. The people check in on their broody several times before going to bed. By morning the chickens wake up together & assume that they've always been together. As with most things it may also depend on the hen.
