Yes, if you have already filed your tax return it is too late to add the additional W-2 at this point. You need to first wait and see if your return is accepted or rejected. If rejected, you can go back and add the additional W-2 and then re-submit your return.
If accepted, you need to wait until you receive your tax refund and then Amend your return using Form 1040X. The form will be available sometime in February.
Here is an FAQ with additional information on Amending: https://ttlc.intuit.com/replies/3288565