If the only reason you have them is to provide feeders then it never makes economical sense to try to fix a broken breeding group. Much easier to replace and be done with it.
In the OP case sure you could keep a female baby in the group and let it grow up to breeding size, but if you did:
1.) it would take FOREVER for the baby to get up to breeding size. don't know why but I have documented that in this exact scenario, the baby takes way longer then usual to get to size.
2.) I have also documented in this scenario that for whatever reasons, often times that baby never will produce babies, and if it does it is likely to be smaller then usual litters at first.
3.) Once the baby finally grows enough, and has a few small litters and starts to have normal sized litters, it is time to retire the rest of the group. So you have effectively wasted a huge amount of time, and the living space that has not produced the number of babies each space is supposed to produce.