adding another female mouse to breeding group

[serpenteye]

I have a small group of breeder mice that just popped out a bunch of babies. This group has been together for many months and they all know eachother very well. Today i decided that i needed to get another female to add to the group. But i dont think that they are going to let her live with them. She is pretty young still, so what would be the best way to add her into the group of mice without her getting killed or hurt?

(The group i have allready are two females and one male.)

Thanks very much.