There is always a serious risk adding a new female to a breeding group. On the few occasions I tried it, the new girl was found weeks later with her face eaten off...so clearly it wasn't all that happy for them.

However, I have had success in making new groups of adult rats by completely mixing them up--if you have just 1.1, remove the male and replace him with a different male, and add the new girl at the same time.

As long as ALL the rats in the new group are strangers to each other, they seem to work it out and get along much better.