If you don't care who the father is, there is no need to separate them for mating -- there's likely plenty of that going on already since you say they all run together.
It sounds to me like you are asking how to get a hen to go broody. I don't think there is a sure fire way to do that.
It sounds to me like you are asking how to get a hen to go broody. I don't think there is a sure fire way to do that.