Can I add another W2 after iI finish my tax?

Yes, as long as you have not already filed (haven't efiled or filed by mail.)

Does TurboTax think you are finished, and you need to know how to open your unfiled return back up to add the additional W-2?

If you have already filed, then that's a different story.  We can tell you what to do.

If you have already filed, not right away.  You would need to wait until your return is either rejected or approved.  

If it's rejected, you can return to the program to add the new information while fixing the error. 

If your return is approved, you would need to amend if adding the new information changes your original refund amount or tax due.  To amend, please click here 2018 Amend.  Follow the instructions in the program and it will tell you if the correction changes the result.  If you do need to amend, please wait until your return is fully processed and you have received your refund or your payment has cleared.