I e-filed my wife's taxes, and after they were accepted by both, I realized I forgot to add a W-2. How do I add it?

You will have to file an amended return.  You should not file an amended return until you have received your refund (if you are due a refund).  If you are not due a refund, make sure any payment you may have made has been processed.  For information on how to file an amended return see the following TurboTax article:

How to File an Amended Tax Return with the IRS