I filled my taxes with my wife but I forgot to add her W2 and my federal was accepted already. Can I still add hers ?

If the return has been accepted you can amend it. But wait until the return has been processed and any refund or tax liability settled before amending. 

it's highly likely that when your joint return is fully processed, the iRS will catch the omission and either adjust accordingly or send you a notice to approve of including her W-2.