dependent visa extension out of japan

I am staying in japan on a work visa with my wife who possess the dependent visa. Since my wife wife is pregnant we are planning the delivery of baby in our home country. But, the problem is that during the period she will be out of japan, her visa term will be over.
If any body can advice me, what would be the process to get her visa extended without her being in japan. And what will the requirements to get visa for the new born baby.
Will it be of any help if my wife takes the re-entry visa before leaving japan. As i have seen in internet that re-entry visa has no utility if the visa term gets over.
Kindly suggest.

I have never heard of anyone extending their visa while outside on Japan. Usually you are not even allowed to leave the country while your extension is processing.

On the off chance that it is possible, the Japanese embassy in your country will have the most information on this.