Can I return to add a W-2 later if I want to continue with the W-2's that I already have?

Yes as long as you have not filed your return. To do this you would follow the instructions below:

Make sure you are logged into your account

Click Federal on the left tab

Click Wages and Income on the top tab

Scroll down until you get to Wages & Salaries 

Revisit the Wages and Salaries Section 

And here is where you will be able to add the other W-2 that you received.